I'm trying to measure a large source impedance using the LTC2440 Dev Kit (DC570A) and the program to control the ADC, QuikEval K113, but I'm getting an awful amount of noise.
For testing I'm using 3x 100k resistors in series and measuring the second resistor. (see diagram below). Also if I increase the 3x resistors (e.g. 1M), the noise increases significantly.
I've tried to use the suggested buffer in the DC570A datasheet and figure 19 in the LTC2440 datasheet. I also tried a simple voltage follower.
Test results (4,5 ms conversion time - approx. 200 SPS, 3x 100k resistors in series):
try using lower value resistors. The resistors themselves will generate noise. The sine wave you are seeing maybe 60Hz.
If I change the sampling rate to a bit lower value (18 ms, I think) the noise clearly looks like a sine wave. So it's pretty clear that the op amp is unstable. But I'm still trying to figure out how to fix it...