Some EE opinions please

[Incoherent]

I'm building a 12bit multichannel thermometer/controller using thermistors and a MAX186 amongst other things.
I need opinions on the analog side. The MAX186 datasheet (http://pdfserv.maxim-ic.com/en/ds/MAX186-MAX188.pdf) will not tell me the input impedance of the analog inputs.
I want to use a voltage divider with one precision resistor and a thermistor between Vref and ground.
It is advantageous to use higher resistance (100K-1M) thermistors because there is less selfheating, but that leads to the question.
Would I need to buffer the inputs to prevent the ADC itself affecting the reading?
Thinking of a LM339 as a voltage follower, is this a stable solution? i.e. will it oscillate?
I'd like to avoid avoid extra circuitry.



Cheers

Incoherent

[Groth]

They don't give an input impedance because it doesn't have one in the traditional sense. It periodically charges a sample-and-hold capacitor: during a three clock tick sampling period, input impedance is very low ~5K; the rest of the time, impedance is effectivly infinite. So the net current drawn by the ADC inputs depends on the sample frequency, and the convertion accuracy depends the host clock and on higher current during the sampling period.

For your thermistors, bandwidth doesn't terribly matter. So the easiest with them would be to put a 10nF cap between the input and ground. The cap would supply the higher currents for the sampling phase, and the net current through your thermistor voltage divider could be kept low enough to not affect the results by cutting the sample rate to 10/s or less.

If you need to measure thing with high bandwidths, you'll need voltage followers on the ADC inputs.

[Incoherent]

They don't give an input impedance because it doesn't have one in the traditional sense. It periodically charges a sample-and-hold capacitor:

Ah. Thank you Groth, an long unused section of my brain suddenly kicked back to life. I think it would probably be safer to use a voltage follower. My intention is to only sample at 1s but that might increase later. It'd let me not worry about the thermistor current. Although low component count is a priority, accuracy is more important.

[Groth]

Yeah, the voltage follower will allow the board as a whole to be more flexible, making it simpler to switch a channel to a different task.

Sounds like a interesting project.

[Incoherent]

Sounds like a interesting project.

I hope so. I am setting up a poor mans die simulator using thermistors and commenced circuit designing when I found almost exactly what I had drawn up.Here (http://www.quasarelectronics.com/3118.htm) but a different ADC. I will use the software and elements of the design. 8 analog channels (thermistors, flowrate, pressure), 4 digital inputs, and 4 output bits to control power level flowrate etc.
I like thermistors, they are easy and more sensitive than thermocouples, RTDs etc so I hope the repeatability will be enough for a good calibration. Will keep me occupied.
Damn I need a time machine.

[Groth]

That's a neat little kit they've got. Seems their pdf's are broken though (or my reader is).

Time machine?

[Incoherent]

That's a neat little kit they've got. Seems their pdf's are broken though (or my reader is).

Time machine?

Simply that it'd allow me to do what I need to do simultaneously with what I want to do. Currently I am too busy so this little project needs to be on hold for a few weeks.

The PDFs are working for me.

[firtol88]

...Here (http://www.quasarelectronics.com/3118.htm)...

Nice find.

[dnkroz]

Hi.

Incoherent, you can also use integrated circuits like the LM35 or similars. It has a very linear answer and it is not necessary to have voltage stable.

The adaptation is relatively simple. I have a similar ADC almost finished.

[Incoherent]

Hi.

Incoherent, you can also use integrated circuits like the LM35 or similars. It has a very linear answer and it is not necessary to have voltage stable.

The adaptation is relatively simple. I have a similar ADC almost finished.

Hi dnkroz
Thanks for the suggestion, I have considered it. The only problem with the LM35 type sensors is their size and shape. I want to insert the sensor into a small (~1mm) hole. Thermistors are available in a very small package.

[DeadEye]

Hi dnkroz
Thanks for the suggestion, I have considered it. The only problem with the LM35 type sensors is their size and shape. I want to insert the sensor into a small (~1mm) hole. Thermistors are available in a very small package.

A few points that may/may not be useful:

You mention accuracy is important, the MAX186's internal reference depending on version can be as bad as +/- 80ppm. If using this chip ensure you get the best version. Much better external references are available.

Incorporating switchable gain instrumentation amplifiers into the design will allow various ranges to be monitored at better resolution.

Linearisation has to be incorporated in the software to handle the thermistor, if working over a shortish range then dependant on accuracy you are trying to achieve a small PRT, could be used which is almost linear over limited ranges.

Consider setting up a small current source (100uA) for powering of sensors, PRT, thermistors or as a resistor measuring device.

Hope this helps.

[Incoherent]

A few points that may/may not be useful:

You mention accuracy is important, the MAX186's internal reference depending on version can be as bad as +/- 80ppm. If using this chip ensure you get the best version. Much better external references are available.

Incorporating switchable gain instrumentation amplifiers into the design will allow various ranges to be monitored at better resolution.

Linearisation has to be incorporated in the software to handle the thermistor, if working over a shortish range then dependant on accuracy you are trying to achieve a small PRT, could be used which is almost linear over limited ranges.

Consider setting up a small current source (100uA) for powering of sensors, PRT, thermistors or as a resistor measuring device.

Hope this helps.

Appreciate the input Deadeye.
I was planning to use a MAX6241 voltage reference via a OP484 type voltage follower buffer to feed both the ADC and the thermistors. Settling on 100K thermistors and 80K 0.1% precision resistors to minimise self heating and lead resistance effects, buffered by said OP484 voltage followers. You sound like you might be able to say if this is a reasonable approach or not.
Linearisation and calibration will be in software (Excel, heh)
Tell me more about these intrumentation amplifiers, I've been looking at the AMP02 but am not sure if I need/can even extract from noise the extra resolution. Plus I want to keep the single rail if possible.
I'm over 15 years beyond studying this stuff.

[DeadEye]

The approach you are looking at seems fine to me, you didnt explain what the whole design is for. If you are just making a basic temperature monitoring sytem covering a limited range only, then I would stick with what you have now. If you are designing a more flexible system for doing other things then generally you need to add optional gain to the circuit.
I know you realise that 4096mV is 1mv per step resolution, so it is easy to work out what you get on your thermistor in this application, however if great ranges are required then instrumentation amps start to offer the benefits. I preffered the Burr Brown (now TI) range due to their low power requirements.
Instrumentation amps offer benefits in precise gain, common mode rejection and ability to run from dual/single supplies. It is also possible to get the inputs/outputs to go to within 100mV of the rail supply.
Just one other comment, I take it you are running fairly short cables for the thermistor ?
If running longish cables then consider the use of constant current supply rather than a voltage divider.

[dnkroz]

I´am using the OP177 to amplify the Vout of the LM35. The obtained resolution is very near to 0.01ºC (0.0001V) with a ADC of 10Bit, 0-2.5V. Without the amplification the resolution is pproximately 0.06ºC

In the measurement of CPU temperature is a relativity problem because the measurement scale is reduced to 40ºC, but for the rest of measures (11 channels) of the system it would be sufficient.

The solution would be obvious to reduce the resolution in this channel (smaller amplification) or to reduce a fixed tension. For example -150mV (15ºC), it would increase the scale of measurement.

Here (http://www.devilmaster.org/forums/viewtopic.php?t=472&postdays=0&postorder=asc&start=0) some images can be seen about my system, and this link NTC (http://www.thermometrics.com/assets/images/ntcnotes.pdf) NTC2 (http://www.thermometrics.com/htmldocs/proserv.htm) if possible that interesting for you.

Still I have left tests to do. Sorry my bad english

[Incoherent]

Can anyone suggest a stable, simple 12v>5v voltage regulator circuit.
Currently I am leaning towards simply a LM317 with precision resistors. I am uncertain of the temperature stability of this and would like to lock it to the 4.096v voltage reference I am using as reference for ADC and thermistors.
I am anticipating that the 12v in will not be particularly stable, it will probably be powered from a hard drive connector initially. Any thoughts on whether power supply stability is even necessary in this case.

The whole circuit (http://w1.863.telia.com/~u86303493/Computer/diesim/DAQ_schematic_incomplete.gif) is based heavily on this (http://www.electronic-kits-and-projects.com/kit-files/3000/3118.pdf) but with a higher end analog side. I am intending to use the same software or a variant.

dnkroz, Deadeye, a belated thanks for your inputs, this thread dropped off my radar over the holidays.
I have decided not to use instrumentation amps at this point, the accuracy I can reasonably expect is going to make the 0.025 deg C resolution I am expecting quite adequate.

[bigben2k]

How about this:
http://www.analog.com/Analog_Root/productPage/productHome/0,2121,AD1584,00.html

or do you need some power to go with it?

Do you need it lower to cover the 317's voltage drop?

[Incoherent]

How about this:
http://www.analog.com/Analog_Root/productPage/productHome/0,2121,AD1584,00.html

or do you need some power to go with it?

Do you need it lower to cover the 317's voltage drop?

Simpler than that Ben. I already have a Voltage reference, (MAX6241) what I need is a regulator circuit for the power supplies. I want this to be very stable, and I was wondering if it was possible to feed the voltage reference to the V adj pin of the 3TR in some manner where it was still regulating properly but was locked to the reference voltage. i.e if the reference drifts, so does the rest of the circuits power supplies, (to amps, sensors and ADC) thus cancelling any error this would generate. I realise the ADC will do it's thing relative to its ref V but I want to reduce the uncertainty.
Is this necessary? The voltage followers would (should) not be affected by Vcc fluctuations either but I am integrating a shunt with an amp for measuring current which might be.

[bigben2k]

Last I checked, a DAC's Vref should come from a seperate source.

I think you're trying to steady the supply voltages for the whole circuit, if I read you correctly. The diagram seems to have the 317 dictating output voltages which feeds Vsupply for all the other chips.

Last I checked, all these components work within a certain voltage range, i.e. 4.5 to 5.5 v. So you'd be looking to eliminate the transients. You can do that simply by adding caps right at the Vsupply and GND pins of each IC. An oscilloscope would give you a detailed picture. A standard PSU would have transients in the 1kHz to 10 kHz range (from the switching), and might have some around 60 Hz (from AC).

Does that help?

[Groth]

The LM317 will be sufficient. Add a 10 uF tantalum cap parallel to R8, a protection diode anti-parallel R7, another diode from LM317 terminal 2 -|>|- 3, make C3 a 0.1 uF ceramic, and make C4 a 1000 uF aluminum.

Couple that with good bypassing of the ADC's V+ (a 22 uF tant and a 2 nF ceramic in parallel, as close to V+ as possible) and you'll be fine.

Have you thought about feeding your thermistors with a larger voltage? Doesn't seem that you'll be getting the full benefit of your 12 bits/4.096 range.

I'd also add protect diodes to your MAX6241, pin 6 -|>|- 2 and 3 -|>|- 2.

[Incoherent]

Thanks for the bypassing/protection suggestions Groth. Gives me solid numbers for values. I will stick with the LM317.




Have you thought about feeding your thermistors with a larger voltage? Doesn't seem that you'll be getting the full benefit of your 12 bits/4.096 range.



Yes I have. Two problems I see.
1. I would need a -ve supply to counterbalance the higher voltage and keep the operating level within the 0-4.096 range, if I wanted to keep the circuit as it is.
2. I'd like to keep the thermistor voltage tied to the same reference as the ADC, both the higher V+ and the V- would not be if I wanted to minimise component count.

A possible solution is this. I back track and DO use intrumentation amps.

Would this or a variation work?

[Groth]

Yeah, that variation would work. You'd definitely want intrumentation amps to avoid input bias current errors. Maybe use a single voltage divider with a voltage follower amp to supply all your channels.

If you want to stick to your present amps and have the thermistor feed voltages dependent on the Vref -- bring in a -12V line, then use an inverting amp with a bit of gain to derive an appropriate negative voltage from Vref, and a non-inverting with a bit of gain to get the postive.

[Incoherent]

Yeah, that variation would work. You'd definitely want intrumentation amps to avoid input bias current errors. Maybe use a single voltage divider with a voltage follower amp to supply all your channels.



Good idea, I think I'll go with this.


If you want to stick to your present amps and have the thermistor feed voltages dependent on the Vref -- bring in a -12V line, then use an inverting amp with a bit of gain to derive an appropriate negative voltage from Vref, and a non-inverting with a bit of gain to get the postive.

Another good, simple idea, you really are full of them aren't you. I like not having to use another supply though so...

[Groth]

Well. if you want to stick to single supply, you could use a charge pump to generate the small amounts of negative voltages you would need...

But, I do like the first variation and the instrumentation amps will give you better signal-to-noise, common-mode-rejection, etc. If you can, pick a low power single supply amp and run it off your Vref (may require a beefier voltage follower); that way the ADC inputs are guaranteed be to be with bounds.

Usually when someone calls me full of it, they're talking about something else. :D

[Incoherent]

If you can, pick a low power single supply amp and run it off your Vref (may require a beefier voltage follower); that way the ADC inputs are guaranteed be to be with bounds.



Something like this (http://w1.863.telia.com/~u86303493/Computer/diesim/DAQ_schematic_nearly.gif) is what it is turning into. A bit of a Hydra. Cut out one component, two others grow back.

Two high resolution (<0.005 deg) low range channels for water In/Out, 4 low resolution (<0.025 deg) for three sensor fluxblock and die. One ammeter channel and one voltmeter channel. I think I am going to skip the digital in/outs but we'll see.

What a mess. Now to see if I can transfer this onto a PCB.

Opinions/corrections please.

[Groth]

Hah, looks like you're designing a plate of spaghetti! What software are you doing the schematics with? They look great.

Generally looks good to me. What sort of the currents are you running through your heater? Might be better to move your current sensing shunt off board.

If you don't do the digital ins and outs yet, how about leaving yourself solder pads or a header that gives access to +5/ground/p-port lines. That way you could attach a daughter card later.

:cool:

[Incoherent]

Hah, looks like you're designing a plate of spaghetti! What software are you doing the schematics with? They look great.

Generally looks good to me. What sort of the currents are you running through your heater? Might be better to move your current sensing shunt off board.

If you don't do the digital ins and outs yet, how about leaving yourself solder pads or a header that gives access to +5/ground/p-port lines. That way you could attach a daughter card later.

:cool:

It's free software from ExpressPCB (http://www.expresspcb.com/ExpressPCBHtm/Download.htm). Quite OK, you can link the schematic to the PCB layout. I'm still learning that part.

The heater current will max out at about 8-10amps, probably a little high to be on board but I am a little worried about connection resistance buggering up the readings. What do you think? The currents around this circuit are very low (10-100µA area), probably a prime candidate for a bit of inductive crosstalk. Any thoughts on reducing this if I did keep it on board. It is all effectively DC of course.

You are right regarding digital i/o, it'd be a bit silly to not give myself at least the future possibility.
I am debating now whether to optocouple the digital side. The added circuit complexity does not appeal.

Been looking at precision resistors, they are not cheap (~$3 each here in Sweden) for 0.1% <5PPM/*C metal film. I'm wondering whether your frypan sorting technique will be applicable here if I used the far cheaper 1% 50PPM/*C ones.

... If you can, pick a low power single supply amp and run it off your Vref (may require a beefier voltage follower); that way the ADC inputs are guaranteed be to be with bounds.
Been thinking, actually theres no real reason why I can't use a 5v reference, If I am reading the MAX186 spec sheet properly, that way I can power the INA122s from Vcc.

[Groth]

The heater current will max out at about 8-10amps, probably a little high to be on board but I am a little worried about connection resistance buggering up the readings. What do you think? The currents around this circuit are very low (10-100µA area), probably a prime candidate for a bit of inductive crosstalk. Any thoughts on reducing this if I did keep it on board. It is all effectively DC of course. With the currents from the sense shunt to the amp so low, connection resistance isn't a big issue there (mV signals, µV connection drops). As long as you use twisted pair from the shunt to the board (to avoid differential noise), just let the in-amps reject any common mode noise that shows up.

You are right regarding digital i/o, it'd be a bit silly to not give myself at least the future possibility.
I am debating now whether to optocouple the digital side. The added circuit complexity does not appeal.Give your self the expansion possibility, and put aside digital thoughts for another day.

Been looking at precision resistors, they are not cheap (~$3 each here in Sweden) for 0.1% <5PPM/*C metal film. I'm wondering whether your frypan sorting technique will be applicable here if I used the far cheaper 1% 50PPM/*C ones.I think you'll do fine without the precision ones. The thermistors will all want to be calibrated anyway, so the 80K's don't need to be precision. The juctions of R13/R15 and R11/R12 can simply be measured - no need for precision. R9 would be cool to have precise, but there's no point in having it more precise than R10...

Go cheap.

Been thinking, actually theres no real reason why I can't use a 5v reference, If I am reading the MAX186 spec sheet properly, that way I can power the INA122s from Vcc. I didn't read the datasheet closely, so :shrug: Check to see how the ADC inputs respond to voltages higher than Vref. Most have a limit to how much over-voltage current they'll sink, be sure the caps on the inputs won't supply more that that.

[Incoherent]

With the currents from the sense shunt to the amp so low, connection resistance isn't a big issue there (mV signals, µV connection drops). As long as you use twisted pair from the shunt to the board (to avoid differential noise), just let the in-amps reject any common mode noise that shows up.

This I'll do.


Go cheap.


Indeed. At this point thanks to Maxims fantastic samples policy, it's definitely looking to be pretty cheap. The thermistors are going to be the biggest cost.


I didn't read the datasheet closely, so :shrug: Check to see how the ADC inputs respond to voltages higher than Vref. Most have a limit to how much over-voltage current they'll sink, be sure the caps on the inputs won't supply more that that.

Analog Input Range and Input Protection
Internal protection diodes, which clamp the analog input to VDD and VSS, allow the channel input pins to swing from VSS - 0.3V to VDD + 0.3V without damage. However, for accurate conversions near full scale, the inputs must not exceed VDD by more than 50mV, or be lower than VSS by 50mV.
If the analog input exceeds 50mV beyond the supplies, do not forward bias the protection diodes of off-channels over two milliamperes, as excessive current will degrade the conversion accuracy of the on-channel.
The full-scale input voltage depends on the voltage at VREF. See Tables 1a and 1b.

As I read this I think I'll be OK if I feed up to Vdd. I gain nothing resolution-wise from a 5V reference I realise, except a little bit of noise sensitivity.

Is the U5/U6 -ve input going to need an input bias current path? The voltage follower o/p is low impedence but would I need an e.g. 40K to ground (I'm switching the 80k-ers to 40k, better linearity in the 20-60 deg range.)

BTW, Thanks for all your input Groth, it has helped/is helping immensely. Re-learning stuff as I go. :)

[Groth]

Originally Posted by MAX186 Datasheet:D Cute.
Uh yep, sounds okay. Use the +5.

Is the U5/U6 -ve input going to need an input bias current path? The voltage follower o/p is low impedence but would I need an e.g. 40K to ground (I'm switching the 80k-ers to 40k, better linearity in the 20-60 deg range.)The low impedance of the follower is current path aplenty.

Thanks for all your input GrothNo problem. When you get the board laid out, you're gonna post it right? We want to mooch! :)

[Incoherent]

...mooch!

Mooch?

:)

[Groth]

It's free software from ExpressPCB (http://www.expresspcb.com/ExpressPCBHtm/Download.htm).
Not a bad little program. It's a lot easier to bung up a tolerable schematic with it than with the SPICE programs I have. And their PCB layout is easy and straitforward. It ain't very powerful, but the learning curve is much nicer than with the hardcore PCB programs I've failed to master.

It's a shame the output is so crippled; they certainly want the software used only with their services. Are you planning to have them make your board? Got to be a way to hack the output...

[Incoherent]

I'll be totally honest, I didn't realise it was a propriatory format, was just happy it was so user friendly. In any case, for me it's a great help just to get the layout, so I can do it manually.
I might use them. The little miniboard for $51 for three boards is not cheap but in the long run (actually in the short run too) it would save a hell of a lot of hassle. That way anyone else interested could perhaps benefit as well. We'll see.
Been thinking about incorporating a radiator fan speed controller into the circuit. As it stands I have a spare amp so I could take a feed from one of the INA122s and feed a fan driver. I am certain that I am able to see quite strong water temperature dependency on WB performance just with the crap thermometer I have now, I'd like to explore this and a bit of water temperature regulation could be useful.

Attaching a near final schematic for general perusal.
The fan driver is incomplete, I'll add a bit of proportionality and hysteresis later. But you get the idea.

[Groth]

Nice to see I'm not the only one who grabbed the schematic. ;)

Looks good. I like how you've used the control/data lines, should make the interface program easier. Let not forget the vital green LED!

If the time constant for your system is large enough, you could get away with just the simple on/off thermostat fan control you have shown, though I wouldn't mind seeing a PWM controller attached to your newly tasked error amplifier. :cool:

What sort of plans have you for the digitals ins and outs?

[Incoherent]

Nice to see I'm not the only one who grabbed the schematic. ;)

Looks good. I like how you've used the control/data lines, should make the interface program easier. Let not forget the vital green LED!

If the time constant for your system is large enough, you could get away with just the simple on/off thermostat fan control you have shown, though I wouldn't mind seeing a PWM controller attached to your newly tasked error amplifier. :cool:

What sort of plans have you for the digitals ins and outs?

Re control/data lines, I really can't take any credit for this, it's a carbon copy of the original (http://www.electronic-kits-and-projects.com/kit-files/3000/3118.pdf) circuit this is based on.
Vital green LED indeed. :) A side effect is that it keeps a 10mA load on the 317, but the blinky lights factor is of paramount importance. ;)
PWM. I'd like to do that but I'm wondering if that might be a bit noisy. ?
No real plans for the digital side, just a nice to have thing. Perhaps for switching the heater power level or multiple pumps.

[Groth]

Audible noisy or electrical noisy? For electrical it wouldn't be a big deal, assuming you have J4-4 be your PWM signal and have the power transistor and, more importantly, the fan's ground off board.

[Incoherent]

Audible noisy or electrical noisy? For electrical it wouldn't be a big deal, assuming you have J4-4 be your PWM signal and have the power transistor and, more importantly, the fan's ground off board.


Electrical. OK, suggest a good PWM circuit. Emphasis on noise reduction and simplicity.
Wouldn't I need some common ground connection? Oh. Not with an optocoupler.?

[Groth]

I guess you could use an opto, but not really necessary. I'm assuming you're going to have everything with a common ground, just without the fan's current return through your board. Given the small but real /trace/connection/wire resistance, you don't want the pulses of fan current to induce noise in your board's ground.

As for the circuit, a linear ramp sawtooth wave is easy to make with a 555 timer. Then run that into a comparator along with the output of your error amp (variable gain on that would be cool), voila PWM.

[DeadEye]

Looks like you both have been busy lately. I like how it has progressed since our last chat.
I have been very tied up with work recently, so hadnt been about much myself. I will go sit back on the sideline and give any comments/input as required but it all seems to be in hand.

BTW, I use the Ranger2 PCB program for my circuits and layouts, it gives every type of output that I would want including photoplot and gerber files for hole drilling etc.

Later guys.

[Incoherent]

Guys.

Regarding grounding of the analog side of things, do you think that a ground plane is a good idea. I am planning to seperate digital and analog grounds, connecting them only at the ADC. Tips? Suggestions?

[Groth]

Separate digital and analog ground is probably unnecessary, since the internal ADC convertion clock (1.7 MHz) is much higher that your comms clock (~1-10 KHz) and your sample rate (10 Hz?). If you really want 'em separate, use the p-port ground as your digital ground and your power connector solely analog.

Plane: yeah, cool if you can. I'd try to lay everything out as a single sided board (easier to home make) and then use the other side as a ground plane.

What sort of through-hole to surface-mount ratio are you going to have?

[Incoherent]

Separate digital and analog ground is probably unnecessary, since the internal ADC convertion clock (1.7 MHz) is much higher that your comms clock (~1-10 KHz) and your sample rate (10 Hz?). If you really want 'em separate, use the p-port ground as your digital ground and your power connector solely analog.

Plane: yeah, cool if you can. I'd try to lay everything out as a single sided board (easier to home make) and then use the other side as a ground plane.

What sort of through-hole to surface-mount ratio are you going to have?

Well, as it is evolving the board will be two sided. I will probably use the ExpressPCB service. I can see that there is no way I am going to be able to lay this out by hand and keep a decently small board.
As it is so far I am managing to keep the bulk of the analog stuf on on side with a few power and digital lines on the other. This means there is a fair bit of room for a ground plane.

The only surface mount stuff are the opamps and the instrumentation amps.

Have a gander at the current unfinished but nearing completion state of things. Right click and save target. Circuit (http://w1.863.telia.com/~u86303493/Computer/diesim/DAQ_schematic3.sch ) and pcb (http://w1.863.telia.com/~u86303493/Computer/diesim/DAQ_PCB3.pcb). You need the ExpressPCB software to view it and feel free to be critical.

Not pretty, I am not giving up my day job. I decided to try the layout for size before seeing if I could incorporate a PWM generator in the available "miniboard" space. It will probably work, I will at least put the traces in.

[Groth]

Uh, both of those links are schematics, no PCB layout. :confused:

I did eventually find a layout: damn fine start, though it's a shame not to have the schematic that's linked to the PCB. Too bad the program doesn't include templates for mounting resistors/diodes vertically - could save you a bunch of space.

You like the through-hole, eh? If it was mainly surface mount and most of the complex artwork was on one side, I could've etched it for you...

[Incoherent]

Uh, both of those links are schematics, no PCB layout. :confused:

I did eventually find a layout: damn fine start, though it's a shame not to have the schematic that's linked to the PCB. Too bad the program doesn't include templates for mounting resistors/diodes vertically - could save you a bunch of space.

You like the through-hole, eh? If it was mainly surface mount and most of the complex artwork was on one side, I could've etched it for you...


Sorry about the mixup, was in a hurry.
The relevant files are DAQ_xx3.xxx. The PCB should be linked to the schematic.

I am leaning heavily towards getting it etched for me. There's a catch though, $51 for three boards. $44 shipping to Sweden. I am not feeling rich.


I do not have much/any experience with surface mount stuff. Is it easier to hand solder? I don't have any special SMD tongs or anything.

[Groth]

Most surface mount part are easy, I prefer them cause I don't like drilling holes. For resistors/caps (0805 is my size of choice), I grab them with plain old forceps (I'm grounded, of course), position them, and touch one end with a fine tipped iron that has a tiny blob of solder on it. Assuming I didn't twitch at the wrong time, the other end gets a similar blob of solder.

IC's are only a bit tougher. Grab one lead with the forceps, position the part, and tack down a corner lead. Then tack down the opposite corner. If you use a decent amount of rosin flux, you can then sweep a heavily tinned iron across the leads, soldering an entire side at once (the flux plus the solder surface tension will prevent bridging; if you do get bridging desolder braid clears it right up).

Well... the SOIC ones easy at least. If you get into TSSOP or QSOP it's more fun.

44 for shipping? :eek: That's absurd.

PCB is looking good. I'm impressed that you've needed so few vias. I think you could clear up some of the loopy paths by swapping which amp in the quad packages is doing what. Same with parallel port.

Your analog (power connector) and digital (p-port) ground: keep 'em separate. Star ground > ground loops. Same with the shunt inputs, don't tie it into board ground.

By the by, the tons of through hole stuff doesn't mean I can't make the board. It mostly means drilling your own holes. Oh, and no plated vias in home boards; components could need soldering both top and bottom.

[Incoherent]

Your analog (power connector) and digital (p-port) ground: keep 'em separate. Star ground > ground loops. Same with the shunt inputs, don't tie it into board ground.



Do you mean totally seperate analog and digital grounds? i.e. connect only the DGND pin of the ADC only to the parallel port ground? No connection to the AGND pin or power supply ground at all? Anywhere? Should the Flipflop and the digital IO's be connected to P-port ground? but at the same time they need a power ground.
Or should there be a single connection for the digital circuitry as per the datasheet...
http://w1.863.telia.com/~u86303493/Computer/diesim/max_ground.jpg
But they suggest a star ground.
Sorry, I get confused all the time about this.


PCB (http://w1.863.telia.com/~u86303493/Computer/diesim/DAQ_PCB4.pcb ), schematic (http://w1.863.telia.com/~u86303493/Computer/diesim/DAQ_schematic4.sch ).
Damn, back when I was supposed to know all this I was drunk. I do remember hating to manually insert little via rivets so I avoided them like the plague. It carries through still perhaps.
Hmm, so surface mount is easy eh? I'll have a look at that I think.

Been thinking about the PWM circuit. The comment about the time constant triggered a flurry of calculation which led to a bit of a tiff with the Differential Calculus. I came out of it OK I think. T@t=e^(-t/c*m*(C/W))*(DTi-((C/W)*W)+(C/W)*W is my thought for the day.
Anyway, this comparator fan control circuit is a PWM circuit in a way. The triangle wave generator is the water itself and the fan. Just a really low frequency and a bit of PFM as well. :)
A bit of friday night irrelevance.

[Groth]

Yeah, they thinkin' the same thing I am, just different scales/justifications. The reason behind not connecting the board ground with the parallel port ground is induction, loop = antenna. In the meantime, the parallel port signals are noisy (no reason for the mobo maker to use filter caps on the outputs, in their normal task it doesn't matter). Beyond the loops thing, letting the the signal current return to ground via the parallel port keeps the local ground cleaner.

The latch doesn't need a similar treatment. It's not bothered by noise, and any noise it introduces when activated isn't important since you aren't going to be doing AD conversion at the same time you're setting the latch.

Then again, do I really know and understand the things I think I know and understand? :shrug:

Vias: I've never done the rivet thing, it sounds frightening. Speaking vias, you could kill a number of the ones west ot ADC if you move the Vref line to underneath the ADC (it's DC so it won't be an issue) and remove the reduntant ground connections.

The parallel port has its inputs at the north end and outputs at the south, while you have the digital-in connector to the south and the -out to the north. You could eliminate a couple vias and shorten a lot of traces there.

Ground for U7 and it neighbors is a problem. All the return current from there passes under the ADC; any noise C3 bypasses to ground will travel directly under the ADC. :(

Diff-Eq, ewww. I went to class three times, don't expect me to decypher that. You should patent that combined PWM/FM concept. :)

[Incoherent]

The beginnings of another attempt (http://w1.863.telia.com/~u86303493/Computer/diesim/). PCB5 and schematic5.

This really is time consuming. I can't figure a way to get the digital stuff through any other way so I'm leaving it. I've moved some things arround a bit.
The via's to the west of the ADC are for some decoupling if I find I need it.

[Groth]

Wow, you did make a bunch of changes. Looks damn good. I like the idea of the spare holes for decoupling.

I played with it a bit, to see if the digital lines could be unkinked any. I swapped the functions of J5 and J6 and switched around which pin goes to which parallel port bit. The only thing that might cause a problem (if you're not writing your own software) is the way I switched a couple pins from the 'control' output byte. Easy enought to switch 'em back...

While I was playing I also removes a redundant power trace, ran grounds to U1 and U2, and switched around which output of U2 went to which ADC input.

I got a little carried away, but I really like puzzles. :)

[Incoherent]

Oh, you've been busy. Thanks heaps for that, I like your treatment of the ADC west components, much better than what I had.
If possible I would like to not have to change the program. The intention is to use the ISEE program here (http://www.quasarelectronics.com/3118_software.htm) , it seems to give me what I want (a time stamped data log file) and some limited calibration possibilities. Most calibration will be done offline, in Excel but there is a very simple control language that in the ISEE program that I'm going to try and use as well. Could also be used to control the fan actually.
This means that I do not want to change the IO distribution too much, my programming ability is close to zero and I don't want the time for me to learn to be another implementation delay.
I am rerouting the digital spagetti a bit though, inspired by your treatment of it.

Edit: Looking closer at your version I suddenly realised what you've been driving at. Brilliant. Simple and obvious on reflection. Another one of what you are full of Groth.
But I will keep the ADC steering signals the same as the original, otherwise your suggestions are the bees knees.

[Incoherent]

OK, near final version is done. (http://w1.863.telia.com/~u86303493/Computer/diesim/)
PCB and Schematic 6_g.

The digital stuff has is as you suggested Groth, the only difference being I have reset the ADC control lines to where they were. Not sure if your suggestion was actually correct in this respect, what does p-port pin 17 actually do? Anway, I am happy that the software will work, it's just a matter of assigning channel numbers.
Things added are some through holes for if I later need filter resistors on the inputs and on the ADC Vdd. A matter of trace cutting.
Some verification to clear up the slight remaining confusion. Should I leave the DGND unconnected to the AGND?
Is it worth spreading the ground around the place?

[Groth]

Explains the recent rumbling in my belly... I'm full of bee's knees.

Ah, I had forgotten about that software and their project (with the stupid pdf that still won't work right for me). Handy cool stuff once the back row of p-port signals are wired how they use to be.

So where's that PWM? :D

[Groth]

what does p-port pin 17 actually do?

Should I leave the DGND unconnected to the AGND?
Is it worth spreading the ground around the place?
The control byte is at p-port address+2; /C0 is pin 1, /C1 is pin 14, C2 is pin 16, and /C3 is pin 17. I've no idea why three of the four are inverted.

I'd keep DGND as it is, but reserve the option of solder-bridging it to AGND if things are funky. (I learned that 'keep the options open' from you :) )

Widening the ground under the ADC would be cool, maybe even run a ground trace under it on the top layer. Large scale ground-pours? Eh, up to you. I don't think it'll make a measurable difference, but it won't hurt.

[Incoherent]

So where's that PWM? :D

How about this (http://w1.863.telia.com/~u86303493/Computer/diesim/). DAQ_PCB6_pwm.pcb and DAQ_schematic6_pwm.sch.

Good to have it on board, I don't need to use it initially if I don't want to. Frequency and amplitude settings are component values I'll play around with a bit.
This is final pending no mistakes or better solutions found over the next few days.

[Groth]

A few inconsistancies between schematic and PCB. :confused: As is, the PWM won't work.

Edit: Superfluous exposition elided. :)

[Incoherent]

A few inconsistancies between schematic and PCB. :confused: As is, the PWM won't work.

Damn it. A mistake there. I haven't considered your suggestions yet but the glaring error is that the inverting input to amp4 U1 pin13 should be fed from amp2 U10 pin7 integrator OP. NOT pin 2-5.

U1 is the comparator. U10 is the triangle wave generator. A near exact copy of this (http://www.maxim-ic.com/appnotes.cfm/appnote_number/3201) from Maxim. Only component values will be different.

[Groth]

Hmm, yeah that'll work. Different way than I've seen before, I hadn't though of offsetting/compressing the triangle wave. Cool.

For what it's worth, some combinations of settings for VR1 and VR2 will stop the oscillator.

[Incoherent]

Hmm, yeah that'll work. Different way than I've seen before, I hadn't though of offsetting/compressing the triangle wave. Cool.

For what it's worth, some combinations of settings for VR1 and VR2 will stop the oscillator.

Well, as you've spotted, it lets me use the raw temperature level untouched. No gain stage. Reducing the amplitude of the TW increases the effective gain and the bias is the set point.
Not a very conventional approach I guess but in my Excel model it works. :rolleyes: I think I'll have some fun with settings before I get something stable. :)

[Groth]

Yup, the more I think about it, the more I like your PWM. Compact, subtle.

I retract all the good things I said previously in reference to surface mount ICs. I just spend a couple hours fighting with a 10 lead micro-SOIC monster (0.3 mm leads on 0.5 mm centers). :mad: I hate it when I can't run traces between an IC's legs.

[Incoherent]

Indeed. The first 186 sample that Maxim sent me was one of those but 20 lead. I flapped around with it a bit before deciding that the SO 1.27mm pitch was the smallest I could contemplate. They were good enough to send me a DIP package as well.
I wonder if there really is any real space gain from using these small chips. The external components need to be further away relatively in order for the leads to have the room to flare out and connect. Plus as you say, there is no room for traces between legs.
I guess its all 4 layer and up.

[Groth]

With access to reflow soldering and plenty of plated-through vias, they're great for space saving. Us poor hobbyists have to make due with with crude solding irons and boards etched in the kitchen sink. :shrug: The board I was working on ended up 30x70 mm; it'd be tough to put 130 pins worth of through-hole in that space.

SOIC/1.27 is my favorite. Small, but big enough for a between-the-legs trace. You do know you're required to make something with that SSOP Max186. :D

[Incoherent]

I have decided to bite the bullet and use the ExpressPCB service so now the boards are on the way to me. Unfortunately I ordered the PCB with the PWM trace error in it. Fortunately it is a very easy board fix, the trace in question run right past the pin it is supposed to connect to.

Now I am assembling a bill of materials and would like some opinions on a suitable PWM frequency for driving fans. As I have it it is running between 250 and >1200Hz depending on gain setting. Is this reasonable?

Some raw specs based on current component values:
Voltmeter 0-23.34V, +/- 2.9mV
Ammeter 0-11.9A, +/- 1.5mA
Giving a Wattmeter 0-278W +/- 0.009mW
Accuracy unknown, component values will be 1%.

4 temperature channels
-40-150°C +/- <0.2°C
0-85°C +/- <0.025°C
18-55°C +/- <0.015°C

2 high resolution temperature channels
15-35°C +/- <0.0025°C

Accuracy unknown, subject to component drifts, tolerances, noise control and accuracy of calibration reference. Who knows? I'll see how the oscilliscope, data noise and repeatability look when it's done.

[bigben2k]

Nice work!

Do you have any idea if you could gain more accuracy from having two temp channels measured in differential mode?

Otherwise, I just might join you in this setup. Let us know how it works out, ok? ;)

[Incoherent]

Do you have any idea if you could gain more accuracy from having two temp channels measured in differential mode?



Probably could. Differential inputs would normally be preferable but for the fact that this particular ADC does not use a true differential mode. This pseudo-differential mode I was not keen on messing about with, it can give some snapping problems at certain input values apparently.
Also I did not want to "waste" two ADC channels on a single input.

With a bit of software I could actually use two of the thermistor channels as they are in differential mode and obtain a delta T reading directly, for example for the water sensors. This might be a more accurate way to get the critical delta Tin-Tout for heat-to-water power calculation. Less sensitive to thermistor ref voltage noise. But a bitch to calibrate.

[Incoherent]

The PCBs arrived today. Three of them. $51. yikes. shipping $44. Holy SHITE!! :eek:
http://w1.863.telia.com/~u86303493/Computer/diesim/pcbs.jpg


Also Texas Instruments have an even better samples program than Maxim, I got six INA122's from them, three day shipping from US to Sweden. Amazing.

This mean's that all the Semiconductors which would have cost me in excess of $120 if I had bought them from a local supplier are free samples. More than makes up for the PCB cost.


:)

[Groth]

Now I am assembling a bill of materials and would like some opinions on a suitable PWM frequency for driving fans. As I have it it is running between 250 and >1200Hz depending on gain setting. Is this reasonable?
A little late but :shrug:

As part of testing out a function generator I just got from the junk shop, I hooked it up to give me a dial-a-frequeny PWM. The whole range of 1 Hz - 100kHz worked; the sweet spot for low noise and wide duty cycle range seemed to be ~900 Hz.

[BillA]

be interesting to get a ballpark on the PWM added noise vs. the reduction in fan noise
gonna be different though for all fans apparently

[Groth]

Silence isn't goal, controlling the water temperature is. Not having the fan screech is nice, but having it willing to run at <20% duty cycle is more useful.

Bill, my sample of three fans all had different ratios of turbulence noise vs. PWM noise. Total noise seem to decrease (measured with an uncalibrated ear) with reduced duty cycle, but noise vs. CFM certainly increased.

If control weren't the big issue, I'd say 25 kHz or more. And maybe pulse between 12V and 3V (not ground), so that the fan's electronics would stay powered. Projects for another day.

[BillA]

deleted
10 chars
I don't think it was gibberish, that's what many report - no ?

[lolito_fr]

be interesting to get a ballpark on the PWM added noise vs. the reduction in fan noise
My statement was based on "insufficient data". Was forgetting that PWM allows lower RPM than DC.

[lolito_fr]

(sorry for the threadjack)

groth, my remark was more an off the cuff reaction to Bills' comment - and applying more to the general case of manual speed control. Sure, if you're using some form of PID and want accurate temperature control, then PWM is convenient + efficient and will also get you less non linear distortion.

Backing up my initial statement that PWM is a bad solution if you are a noise freak, I would add that if you need to run your fans at 20% then maybe swapping them out for lower speed versions would buy more dB :) Otherwise just filtering the PWM output with a cap would work, I guess.

[Groth]

Not my thread, hijack away! :D

Yeah, I figured that you (and Bill) were thinking of the general case of speed control. I don't expect everyone to keep track of the the details of every thread, hence my pointing out control was the overriding goal. Never meant to sound harsh or dismissive. I agree completely, PWM isn't for the noise freaks.

To continue farther afield...since the fan motor has some inductance, once you add your capacitor on the output, all you need to add is a freewheeling diode to make a crude switchmode supply. :)

[Incoherent]

An update.

Basically the last few weeks have been spent ironing out little faults, (a bad solder joint, a 2 1/2 year old's intervention which blew the digital circuitry etc)

I have connected the digital ground to the analog ground. There was an offset in the raw output of about 4-5 LSB with it disconnected with noise levels the same so I figured it should be there.

I have spent a fair bit of time generating Steinhart-Hart coefficients for the thermistor calibration. I have done this by using 0.01 deg C (crushed ice and water), 100 deg C (boiling point) and ~37 deg C (water at about body temperature as measured using a medical thermometer, multiple samples taken as an average). This has generated curves for the four low resolution channels which ensure that they agree to within +/- 1LSB (~0.025 deg C) when taken as an average, over a range from ~20-55 deg C.

I am amazed at how well this unit works. I need to check linearity but I am confident that the S-H calibration coefficients are giving a good result, it is an industry standard method and typically has errors of less than 0.01 deg.

This document (http://www.beta.dk/betathermkatalog/teoridel.pdf) has a lot of very useful info.

I have made a new fluxblock. It has 3 sensors spaced 5mm apart to check the linearity of the measured heat flux. This one is 12x12x12mm, a 10x10x12mm version is planned. I have also made a matching heat die with one sensor. This will allow me to finally measure the TIM resistance with a fair amount of precision by interpolation.

Two water sensors, with a resolution of <0.005degC will let me crosscheck power to water vs power to WB vs Power to heat die. Secondary and tertiary losses will come from these numbers, curiosity only, no practical value, only applies to this setup but interesting to see the effects of insulation.

Still haven't played with the PWM circuitry, it will come.

[bigben2k]

Very cool... rest assured I'm keeping a close eye on your progress! ;)

[bigben2k]

...and sure enough.

I picked up a six-pack of AD7713AN
"CMOS, Low Power 24-Bit Sigma-Delta, Signal Conditioning ADC with Matched RTD Current Sources"
http://www.analog.com/en/prod/0,2877,AD7713,00.html

...and two AD580 and two REF192FS.
http://www.analog.com/en/prod/0,2877,AD580,00.html
http://www.analog.com/en/prod/0,,769_838_REF192,00.html

Now...

I've already got too many things going on (as if no one knew!) so this is probably going to sit on ice for a while, but I thought I'd put the offer forward; if anyone is interested in building a similar circuit, I'll supply the ADCs and the volt references. Heck, I'll even cover the cost of the PCBs.

[jaydee]

Pretty interesting read here. ben linked it from another thread and just got around to reading it. This reminds me of another DAQ based on the maxim186 I cam acrossed called the MiniDAQ. http://www.embeddedtronics.com/Electronics.html

Anyway my question to Incoherent is what software are you using with it. Sorry if I missed it.

[Incoherent]

http://www.electronic-kits-and-projects.com/3118_software.htm
ISEE DAQ software for the kit that the digital side of this DAQ is based on.
It is quite OK functionality-wise but a little limited in data processing power.
I have not had time to set up a Labview reader, but that is the plan longer term, that and moving to 16bits, I'll probably use one of the National instruments DAQs.

[leejsmith]

Incoherent, I have one of the 3118 kits and sucsessfuly put it to gether. I have tried getting it to monitor the volts through a 10k POT so i can adjust the volts but in the software it never changes. If i remove the power the software then shows a drop down to 0v.

Could you do a simple 3118 for dummys guide like me ? All i want to do is data log my cpu , vga , water in , water out and case temp. I dont need to be as precise as your system just to get a good idea of whats going on.

I have 100k bead thermisters for probes left over from a joystick port temp reader project will they be ok.

[Incoherent]

Incoherent, I have one of the 3118 kits and sucsessfuly put it to gether. I have tried getting it to monitor the volts through a 10k POT so i can adjust the volts but in the software it never changes. If i remove the power the software then shows a drop down to 0v.

Could you do a simple 3118 for dummys guide like me ? All i want to do is data log my cpu , vga , water in , water out and case temp. I dont need to be as precise as your system just to get a good idea of whats going on.

I have 100k bead thermisters for probes left over from a joystick port temp reader project will they be ok.


How are you wiring the pot? This might be stupidly obvious but, One end should be at +5V, the other at ground (0V) and you should monitor the voltage at the wiper.
The 100k bead thermistors should be fine.
Put them each in series with 40k resistor a between the +5v and 0v with the thermistor closest to the +5v and monitor the voltage at the connection. The curve will look something like the attachment. To reduce the linear temperature range increase the resistor value.

[leejsmith]

How are you wiring the pot? This might be stupidly obvious but, One end should be at +5V, the other at ground (0V) and you should monitor the voltage at the wiper.
The 100k bead thermistors should be fine.
Put them each in series with 40k resistor a between the +5v and 0v with the thermistor closest to the +5v and monitor the voltage at the connection. The curve will look something like the attachment. To reduce the linear temperature range increase the resistor value.

The 100k Pot works perfect thanks :)

I dont have any 40k resistors how would i use the pot ?

at the moment i put the 100k thermistor on the center of the pot and then to the 3118. all i get is a wave and the amplitude increases when i touch the thermistor.
I am going to look though this thread again i am sure i saw something like this in there.

(edit) i put the thermistor across the pot 5v and the output and it works ok.
I tried running the power of the pc psu and a switch mode psu but they are both the same.

[Incoherent]

The 100k Pot works perfect thanks :)

I dont have any 40k resistors how would i use the pot ?

at the moment i put the 100k thermistor on the center of the pot and then to the 3118. all i get is a wave and the amplitude increases when i touch the thermistor.
I am going to look though this thread again i am sure i saw something like this in there.

(edit) i put the thermistor across the pot 5v and the output and it works ok.
I tried running the power of the pc psu and a switch mode psu but they are both the same.

Is it a 10K or 100K pot? If it is a 10k you need to get a larger value resistor because no matter how you do it it will be very non linear in the temperature range you are working with.
Wired the way you have it your range is limited, you are not using the full resolution that you can achieve. If its a 100k pot wire it as attached, set it at half way and do not touch it. Then you can start thinking about calibrating.
You will find that you can change the value by adjusting the pot, it might feel like a way to calibrate it but it is not, it is just going to destroy the linearity of your readings.

[leejsmith]

they are 100k and they are working ok. I have 4 sensors running but the pots need to be replaced with resistors.

thanks again :)

[leejsmith]

I used this website before for calibrating the thermistiors.

http://www.benchtest.com/gp_Temp3.html

there is a xls spread sheet for calculating the curve of the thermistors. Will the mv reading work in place of the joystick port readings ?

steinhart.xls (http://www.benchtest.com/downloads/steinhart.zip)

http://www.benchtest.com/images/s&h1.jpg

I am trying to find an old program i wrote that read from the joystick port and had the xls functions built in to.

[Incoherent]

there is a xls spread sheet for calculating the curve of the thermistors. Will the mv reading work in place of the joystick port readings ?


Absolutely.

You just need to calculate the resistance of the thermistor at the three given temperatures from your readings. Knowing the +5v, the voltage at the thermistor (your DAQ reading) and the resistance of the series resistor, you can figure this out. (5V-Vtherm)/(Vtherm/R)=Rtherm I think. Plug in the Resistance and calibration temperature for the three temperatures and you're good to go.

Good excel sheet. I did all my calculations myself but I just checked it against that sheet and it give the exact same answers for the SH-H coefficients using a slightly different method.

The ISEE software control language lets you put these equations into it so you can get a calibrated temperature output directly. It's a bit buggy but works, just don't feed it divide by zero problems or it will crash.
A sample might look something like this:

@RES1=(5 + (@CH1/1000)*-1)/((@CH1/1000)/40000) =====> (the thermistor resistance.)
@LOG1=LOG(@RES1) =====> (natural log of the resistance)
@TEMP1=0.00147408 + 0.00023704*@LOG1 + 0.00000010839*@LOG1^3 =====> (A,B and C coefficients, the steinhart-hart equation )
Tags: @CH1 is ADC input, @RES1, @LOG1, @TEMP1 are analog variables

There are nicer ways but this works.

[leejsmith]

i have 4 probes working using 100k@25C thermistors.
I am working on the calibration and have 2 very close but 1 is a little off

this is the calibration temps

http://www.ljsnet.co.uk/temps.gif

I left the probes in the hot water until they cooled down and this is the results.
you can see the pink line is higher then the other 2 even though i calibrated them all at the same time.

Some think not right with the pink reading they are a bit all over the place compared to the others. I wonder if i put the correct ground on this one.

http://www.ljsnet.co.uk/temps2.gif

I have a couple of probes from a lian li 5 1/4 drive bay temp unit. they look like they are 50k at 25C will be getting them working soon so i can use them on the cpu as the end of the probes are very thin comapred to the bead thermistors

thanks for all your help i am working an a vb probram to capture the probe data and apply the curves automaticaly plus have a calibration screen.

[Incoherent]

Looks good apart from the ringing. What time period are you logging at?
Try putting an RC filter on the ADC inputs, i.e. connect a 1k resistor in series with your signal line with a 0.47µF cap to ground after the resistor. This won't affect your average reading and will stabilise the signal, its a lowpass filter.
The temp reading difference might be caused by errors in the real resistance of the resistor in your voltage divider (the one in series with the thermistor). Measure it if you can and adjust the value in the resistance calculation. Otherwise, if there is no major linearity problems i.e. the difference is constant, you can make small adjustments to the A coefficient value to tweak the offset away.


Adding a pic of a clip.

Not relevant, just don't want to start a new thread or bump this one

[leejsmith]

I have this working a lot better after i checked the circuits and found a couple of mistakes. Also i found that if i have a certain power supply switched on the ringing gets worse.
I have the vb program working and can calibrate the reading very easy with it just need to add things like logging to a file or spreadsheet.