Buffer noise

Hi,

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):

Method QuikEval K113 results Op amp
Shorting IN+ and IN- 0.8 ppm (+- 0.4 ppm) N/A
Voltage follower 400 ppm (+- 200 ppm) LM324N
Voltage follower 400 ppm (+- 200 ppm) LT1884
DC570A suggestion (5,6k resistor instead of 5k) 200 ppm (+- 100 ppm) LM324N
LTC2440 datasheet figure 19 (although DC570A has 100 ohm resistor in series) 200 ppm (+- 100 ppm) LM324N

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