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bigben2k · 07-25-2004, 08:30 PM

...and I finally catch up to this thread!

Granted that it's not a normal application. I suppose that I assumed that most products would not have a resolution of 0.001 because such a requirement rarely exists in real world, hence not being able to find such a product.

I realize that calibration for an absolute temp went out the door quite a few steps back at this point, but the target here is to obtain a temperature differential: different setup, different calibration (still fuzzy, but easier to do, I believe).

Someone please point out my flaw in logic:
I'm looking at a 24 bit ADC from TI, model 1217. Linearity is ~0.0012%, one bit accuracy (worse at higher sampling rates).

If I use a transmitter (RTD or Thermocouple), I'm going to add another linearity error, so I want to avoid that. The Texas Instruments ADC (Analog to Digital Converter) has a built-in PGA (Programmable Gain Amplifier), but it also induces a linearity error.

The TI ADC wasn't designed to be used to read temps, it's just made to accept a general input, within a certain range.

In order to read a voltage, I'll need a reference supply voltage, which will also induce an error.

If I go with an RTD, the voltage would probably have to be quite high, because the RTD's resistance changes so little over a temp range (low slope), where an TC changes more radically, but not linearly (if I remember correctly).

The high voltage would exceed the ADC's input, so it would have to be offset, which would probably add another error. I'd have to calculate the TC's voltage range, to see if it needs an offset, but I have to assume that that's the case here too.

Throughout all this, I'm out in the dark about what resolution can actually be extracted out of the temp probe, i.e. the output limitations of the probes, which may extend into the response time. At this moment, I'm assuming that either an RTD or TC temp probe is capable of actually capturing a temperature within a resolution of 0.001, the trick is to get that information. Line noise would be a significant factor at this point, requiring some shielding. Only a test setup (or a knowledgeable person) would tell me what kind of noise I'd be picking up, with various shielding solutions. a 4wire RTD would probably be most appropriate here.

That aside, calibration of a differential temp would be a challenge in itself, I agree. The styrofoam cup-o-water only goes so far.

07-25-2004, 08:30 PM	#10
bigben2k Responsible for 2% of all the posts here. Join Date: May 2002 Location: Texas, U.S.A. Posts: 8,302	...and I finally catch up to this thread! Granted that it's not a normal application. I suppose that I assumed that most products would not have a resolution of 0.001 because such a requirement rarely exists in real world, hence not being able to find such a product. I realize that calibration for an absolute temp went out the door quite a few steps back at this point, but the target here is to obtain a temperature differential: different setup, different calibration (still fuzzy, but easier to do, I believe). Someone please point out my flaw in logic: I'm looking at a 24 bit ADC from TI, model 1217. Linearity is ~0.0012%, one bit accuracy (worse at higher sampling rates). If I use a transmitter (RTD or Thermocouple), I'm going to add another linearity error, so I want to avoid that. The Texas Instruments ADC (Analog to Digital Converter) has a built-in PGA (Programmable Gain Amplifier), but it also induces a linearity error. The TI ADC wasn't designed to be used to read temps, it's just made to accept a general input, within a certain range. In order to read a voltage, I'll need a reference supply voltage, which will also induce an error. If I go with an RTD, the voltage would probably have to be quite high, because the RTD's resistance changes so little over a temp range (low slope), where an TC changes more radically, but not linearly (if I remember correctly). The high voltage would exceed the ADC's input, so it would have to be offset, which would probably add another error. I'd have to calculate the TC's voltage range, to see if it needs an offset, but I have to assume that that's the case here too. Throughout all this, I'm out in the dark about what resolution can actually be extracted out of the temp probe, i.e. the output limitations of the probes, which may extend into the response time. At this moment, I'm assuming that either an RTD or TC temp probe is capable of actually capturing a temperature within a resolution of 0.001, the trick is to get that information. Line noise would be a significant factor at this point, requiring some shielding. Only a test setup (or a knowledgeable person) would tell me what kind of noise I'd be picking up, with various shielding solutions. a 4wire RTD would probably be most appropriate here. That aside, calibration of a differential temp would be a challenge in itself, I agree. The styrofoam cup-o-water only goes so far. __________________ WBTA (Water Block Testing Alliance) Nordic Hardware WC 101 Last edited by bigben2k; 07-25-2004 at 08:40 PM.