Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

RE: Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

DAB — Sat, 11 Nov 2023 20:53:09 GMT

Hi Jan,

I found it interesting that when you used the acronym DAB, I got credit for a mention.

RE: Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

Jan Cumps — Sat, 11 Nov 2023 09:15:48 GMT

These are the first measurements reported by the ADC:

I'm running the board's test software: https://github.com/shabaz123/adc_board_test

RE: Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

DAB — Tue, 07 Nov 2023 20:27:40 GMT

Great work Jan.

RE: Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

ggabe — Sun, 05 Nov 2023 16:17:45 GMT

Little more on accuracy: If we target 3.5 digit, or displaying from -4999 to 4999, the LSB of the instrument is 0.01% or 100ppm. The ADS1115 ADC quotes about the same accuracy numbers: Gain error, Typical: 0.01%, Offset Error Typical +/- 1LSB. If it does not require major sacrifice, I would make the AFE just a little bit better than the ADC, but not significantly better.

Doing so will call for a ADC diving grade op-amp, just like the OPA2325, or THS4531, and resistors with controllable match, perhaps with a trim to max out common mode rejection. I do not see a way how CMR can be calibrated in software, other than with good match on the resistors/trimming. Thermal match is out of the question I think, cost prohibitive. However, we can opt for 100ppm/C or so 0.1% precision resistors, at least to have a know thermal behavior. For example: RC0402BR-07100KL, $0.30 each. We might find 0805 too if we search.

RE: Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

ggabe — Sun, 05 Nov 2023 15:00:56 GMT

The question to me is now much gain accuracy is good enough, for a 3 1/2 digit instrument? We can answer that with an exact number, like 0.1% and +/- 1 LSB.

I think TS9224 is not even close to it. No surprise, as TS9224 is a general purpose RRIO opamp, the data sheet does not even attempt to describe output swing close to rails or offset voltage dependency on the common mode voltage. A better suited opamp for analog frontend in the 5V RRIO category is OPA2325. I used it in my 5 1/2 digit DAQ. It has a well specified and very flat Vos vs Vcm function. Not saying it's a must, just to show a higher end part with no surprises.

https://www.ti.com/lit/ds/symlink/opa2325.pdf?ts=1699132603600

RE: Measure the Analogue Front-end of Data Acquisition Board for Pi Pico

shabaz — Sun, 05 Nov 2023 14:32:09 GMT

Hi Jan,

These are very comprehensive measurements, it's great to see all the detail at each measurement point! This will make it simpler to code for it, now knowing how best to compensate for it (and easier now to suggest a calibration procedure for users, since they would need to apply a voltage that they can confirm with their multimeter).

Thanks for the awesome work.

Also, great testbed! Neat idea to use the breadboard as an interface like that : )