Analog Pins seem affected by shared ground or other RESET connections

Development Tools IDE 1.x
1

I'm not sure if this is the correct forum for this question but it seems to be the closest I could find.
Is there a reason that certain connections on the Arduino seem to interfere with the Analog connections?
That is I have an analogRead() that seems to be affected when I connect another arduino to the same ground. In another case the analogRead() is affected when I connect an external RFID reader's RESET pin to any digital pin.

Any idea why this may be?

2

You would need to supply a schematic and the sketches used to get a better answer.

A layout of the power system would also probably be appropriate.

Also how much of a difference.

I have anywhere up to 7 Arduino's connected at once to a USB powered hub which shares a common ground.
Never noticed a difference between readings if one is powered from its own supply compared to being on the hub.

Clone Arduinos do have a much wider noise band though on analog inputs even with nothing connected to the inputs and they are more sensitive to stray capacitance.