Differential ADC input buffer

Question

I am trying to make 10Mohm of input impedance into a ADC (CS5530). I am planing to use differential input buffer to ADC so I am trying to use TI's document as reference. I tried the one differential in differential out single supply and dual supply. But I am not getting decent signal out of buffer (I am using sinus signal).

Basically I am trying them without ADC putting signal in and look them out with scope. I can't see sine signal out.

I am confused about GND levels I mean I should buffer GND level of signal generator and if it is big from other GND then opamp may broken or may stuck on rail limit. Circuit must consider voltage difference between 2 inputs.

Any suggestion is appreciated about getting 10M input impedance.

Schematic is as follows:

ADC: planing to use CS5530
opamp : LT1013
circuit built on breadboard

Answer 1

In the dual supply schematic, there's no reference for the input voltage. Just as a for-instance, if there's no reference to ground on the input side, then you could be sending in 1000V+1*sin(x) which would result in the op-amps being railed for the entire time. Once it's got a good reference, it will be just sin(x) and shouldn't be railed so long as the resistors are chosen well.

The single supply also has this issue. The input is AC coupled which is fine, but the only place current to go is into or out of the output of the op-amp. If you've already railed it, then it won't be able to move from the rail.

You'll want both circuits to have some kind of path to ground so that when the signal is 0V input, it will level out to 0V output.

RT should be split into two resistors with GND sent to the middle of it. CM on the single supply should likely be connected to ground probably through a resistor.