A previous iteration of my circuit was as follows:
- I am sensing DC voltages across 6 variable resistors (1kΩ Pt RTDs)
- The 6 resistors are connected in series; the series is connected to a current source (LM334 set to 0.28mA) on the high side and ground on the low side (this both provides bias current return path and limits current through each RTD to <0.3mA as required by their spec)
- Each resistor has Kelvin connections to a 12-bit 12-channel track+hold ADC (MAX11617) configured in differential mode
- Some I2C stuff to get data out of the ADC
After prototyping this, I found the ADC readings to be very noisy, which I eventually realized was due to the ADC inputs having a high-impedance (GΩ range, due to the current source), leading to slow charging of the internal track+hold capacitor in the ADC as well as crosstalk between ADC channels, and resulting in signal degradation. I confirmed this by first ensuring that the ADC inputs were stable (they were, with ≤5mV peak-to-peak variation, which is mostly attributable to power supply noise in the prototyping setup) and then bypassing each ADC input to ground a with 0.1µF ceramic capacitor. The output signals immediately stabilized with bypassing.
The time constant of the resulting RC network on the ADC input was in the ballpark of 10s, which may be okay for my application. However, the MAX11617 data sheet recommends buffering the inputs in this case, so I decided to explore that.
In the current iteration of the circuit:
- The series configuration of the sensors and the current source is retained.
- The Kelvin connections from RTDs are now going to instrumentation amplifiers (INA2332, configured with gain of 5 and 0V reference voltage).
- The 6 outputs of the instrumentation amplifiers are going to 6 inputs on the same ADC, now configured in single-ended mode.
- I2C stuff is retained
However — and this is where I am baffled — this also resulted in noisy ADC readings. I again confirmed that ADC inputs (aka amplifier outputs) are stable (≤5mV P2P variation) and then again tried bypassing the ADC inputs to ground with 0.1µF MLCCs. The outputs stabilized with bypassing.
This suggests I am seeing slow T+H charging times again, but now I am confused as to why that is happening; my understanding was that the whole point of using an instrumentation amplifier was that it has a low impedance output, and so I was expecting the ADC outputs to be stable when multiplexing across 6 low-impedance inputs.
PS: Tried making a minimal working example with only 2 RTDs and can't repro the problem. I will rebuild the original example one step at a time until the problem reappears, and I'll be back when I have more info from that.
Schematic of current iteration: