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LP324: OpAmp Output Drifts High

Part Number: LP324
Other Parts Discussed in Thread: LM324

I'm using a couple of the channels of the LP324 as buffers for an incoming DC signal through a voltage divider (image 1 included below).  The OpAmp is powered by +24VDC single supply.  The output of this goes through a low pass filter to an ADC pin on an Atmega16A, whose ADC input circuit is shown below (image 2).  When "SigIn" is grounded the ADC measures there to be around 250-300mV.  When placing a multimeter on either side of "R1" I can watch the signal decay down to around 50mV, which can be confirmed as the ADC begins to measure in kind with the multimeter.  This can also be observed on an oscilloscope in DC mode.  We swapped out the LP324 opamp for something with better specs and the problem went away.  Also, if we put a 100k Ohm resistor in parallel with C1 the problem seems to go away.  If we remove "C1" the circuit oscillates with the sampling frequency of the ADC.

I would like to use the LP324, and I feel like putting the 100k Ohm resistor in parallel with "C1" fixes the issue, but I have not been able to conclude with good reason why this is, given my limited knowledge of opamp specifications.

Could somebody please describe for me why the opamp's output would seem to float up to what seems to be a DC voltage in the range of 250-300mV until a load of some kind (even a multimeter) is applied?  Also, could you point me at the spec in the LP324 datasheet which would help to describe this?  Please let me know if you need further information, I've been at this for a few days now trying to decide why it's doing this and I've done plenty of different tests, I only noted the ones I thought to be the most helpful.