Multimeter capacitance value - ADC readings

[VerifEng]

I have set up my raspberry pi to read from the MAX1204. To know if I'm reading the proper value, I use a multimeter at the input channel of the ADC, and compare it to what I read through my program. This works great. However, when I disconnect my multimeter, I don't read the same value. If I was reading 1.264 - 1.268 (correct measurement), I now read 1.248- 1.252.

I assume it's due to the internal capacitance of the multimeter, or some other reason. How can I set up a capactiance or a way to read the correct readings without constantly needing to have the multimeter plugged in? I tried using a 0.01uF, 0.1uF and no luck.

Block Diagram (to try and replicate schematic):

 

[ronsimpson]

Things are often not what they look like.
The multimeter, (I don't know what it is, but maybe it adds 10megaohms or 1meg or maybe it is an old meter that adds 100k).
Some meters add capacitance. 1000pF If there is some ac you can not see, this capacitance may effect the readings. The ac might be FR or ripple from one of the power supplies. Maybe OP1 or OP2 is oscillating (or on the edge of it) and adding some thing.

Probably touching the "1.26V point" with your finger will effect the readings in a similar way.

 

[VerifEng]

ronsimpson, do you think then that the value's I am reading with the ADC are correct then? I know to base myself off of the digital multi meter to know if the value is correct, but now It seems to be the problem.

I will try and add a capactice of 0.1uF on the voltage divider, and see if it'll help. I'll also try to add a resistance between the output of the second buffer and the ADC to also see if it'll help.

Btw, thank you for all the help you've given me lately.