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Instrument amplifier and open input noise....

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Pcba

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I am still struggling with with AD623 instrument amplifier a little.

In my circuit each input (2+3) is connected to a testpin, which is of course open if not in use (there is a 4K7 in front of each input as protection). The circuit works fine when in use, but when not in use the open pins catch lots of noise and I get an erratic output signal.

How do I tie the inputs down?

The input signal is in the upper µV range, or in other words, I want to measure the voltage drop on a very low mOhm range.
 

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I am still struggling with with AD623 instrument amplifier a little.

In my circuit each input (2+3) is connected to a testpin, which is of course open if not in use (there is a 4K7 in front of each input as protection). The circuit works fine when in use, but when not in use the open pins catch lots of noise and I get an erratic output signal.

How do I tie the inputs down?

The input signal is in the upper µV range, or in other words, I want to measure the voltage drop on a very low mOhm range.

hi,
I fit a 10nf or 47nF capacitor across pins 2 and 3, choose a good quality capacitor.
The datasheet recommends a filter using the 4K7 resistors and the 10nF, this will reduce the noise into the AD623 amplifier.

So that your inputs are not 'floating' when not in use, I use a 2 pin jumper plug mounter on my PCB which I can short out the inputs.
Use the two pin plugs and jumpers you can find on an old PC card.
E

EDIT: Look at Fig 51 of the d/s for the option of using two 100K resistors to 0V, one from each pin 2 and 3.
 
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