CPU Diode calibration: Round 2

[Since87]

Originally posted by pHaestus
How many bananas are we talking about for parts?

Depends.

If the circuit provides an output voltage proportional to the CPU's current draw, I'd say parts cost could be kept between $50 and $100.

If the circuit provides an output proportional to Wattage, I'd guess more like $100 to $150.

But at this point, these are really rough guesses. I think those are conservative numbers, but not enormously conservative. I realized today that I probably have a pile of obsolete circuit boards I can pull some, if not all, of the OPA-2227's from. That could make a significant dent in the parts costs. There may be a lot of other potentially useful parts I can scrounge. I'll have to take a look around at people's junk stashes at work to see what I can get for free.

[Groth]

Did a bit of playing around. I tried various values and types of capacitors across the shunt. The biggest change I saw was with a 1 uF polyester cap; the 2000 sample average for CPUburn's current draw dropped by 0.014%.

Connecting the inverting input bypass cap to the output is a good concept to know. One of my early versions, with the inverting input bypassed to ground, had some ugly feedback problems. I moved/replaced the filter caps as per your suggestion, using 4.7 nF ceramics (largest I had in 0805, which is all there was room for). Results: average current reading down 0.03%, population standard deviation down 0.72%. :shrug:

As for the rest (up too late to quote), I'll be making multiple prototypes and versions, anyway, so, I'm open to doing the building. I don't think there's any need for a watt output - it's easier and cheaper to do the multiplication in software.

Mmmm, laser trimmed resistors...

[Groth]

OK, trying to get a handle on accuracy...

Quite by accident I discovered that when a floppy is left in the drive and the computer complains "non-system disk", it uses a steady current. A current that varies with Vcore and multiplier, but is boot-to-boot reproducible within a 10 mA range. All the switcher noise, non of the software variability.

Using the known gain of my circuit and the nominal LSB value of my ADC, I computed the expected byte values for a series of multimeter readings. I then divided the actual value, as recorded by MBM, by the expected value.

For the statistics crowd, variance comes up as 0.3%. Not bad at all for a 2% ADC and 1% (in this range) multimeter. Unfortunately, the shunt has a 3% tolerance, so overall accuracy still sucks. I wonder how it will fair versus a fully calibrated, precision Since87 design.