ADC buffering and crosstalk prevention?

I require a high input impedance in one of my projects so I've added an OPAMP buffer with a fairly large resistor. The OPAMP is OPA4314 data sheet, product information and support | TI.com. I'm powering it from 5V for a brief period while taking measurements in order to preserve all the power I can. It has about 200ms before applying a voltage to its input. The voltages are usually DC.

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All in all the circuit works as expected but I had noticed that when my hand is close to the IC there is voltage output on other channels of the quad OPAMP. What would be the most reasonable way to overcome this? Also, what could be the reason for this?

Maybe because the other op amp inputs are floating, and they are also high impedance, any electromagnetism changes near them will cause a voltage to appear at the input. Your hand will create slight changes.

My suggestion would be to tie all inputs you are not using to GND.

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Here is the problem... I am using all 4 OPAMPs to buffer different inputs so I need all of them working and I am not sure how I can shunt them to GND using low Z path to prevent them from easily picking up noise. Right now they all see high Z at the input so very low voltages would get picked up. Perhaps lowering the resistor to 500K could help, however I am also losing the high input impedance I was after.

Put it in a metal box and use shielded cable (and short connections if possible) for input-connections going into the box.

Since you body is likely picking-up 50/60Hz power-line "hum" try a higher value for C4 (at least 1uF).

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The A/D of an Uno is extreme high impedance (more than 100Megohm).
And cross-talk between it's channels can be fixed with code.
So why do you use opamps, and for what.
Leo..

The A/D of an Uno is extreme high impedance (more than 100Megohm).
And cross-talk between it's channels can be fixed with code.
So why do you use opamps, and for what.
Leo..

I am actually using a non-Arduino board with an ST ARM Cortex M4