I have been working on and off. Got my prototype PCB back and did some noise and input current testing.
Here is a photo of the current PCB:
I soldered the 4 In-Amps on one at a time and measured their performance, here is a (offset corrected 0.1-10Hz filtered 100sec) plot of the PDF of the shorted inputs:
Some statistics:
| 1 In-Amp | 2 In-Amp | 3 In-Amp | 4 In-Amp |
RMS (nV) | 6.76 | 5.04 | 4.59 | 4.25 |
Pk-Pk (nV) (10sec) average | 40.05 | 32.33 | 31.40 | 31.46 |
Pk-Pk (nV) (10sec) max | 48.66 | 47.07 | 47.12 | 55.43 |
Pk-Pk (nV) (10sec) min | 29.63 | 24.48 | 22.64 | 18.41 |
It is pretty clear to me that adding more than 4 In-Amps is a losing proposition.
I additionally did some testing, just leaving the board in a box, sandwiched in a towel to prevent air movement, and recorded the warmup and thermal drift over time, over a 5C change in air temp, I saw a 500nV change in the shorted voltage:
I did some testing of the input current, between +-900mV it is about 25nA (flowing into the input). As I approach +-1V the input current hits 3mA, which was surprising (though in retrospect not
that surprising). The datasheet only mentions limits in the Absolute Maximum Rating section (which of course I failed to read), so my elegant nV to 10V input design without any switching (except for in the ADC) will not work as planned.