Does each analog input pin affect to others??

Using Arduino Sensors
1

Hello I have a question about Analogread.

For my understanding, each analogread pin is isolated and should not affect to each other.

Please see the picture I draw.

I applied the 2.5Hz AC signal amplitude 100mV peak to peak with 200mV offset to noninverting amplifier.

The first amplifier gain is 10 and the second one is just buffer.

There are three points, P1, P2 and P3.

I measured the peak-to-peak value at each point with Analog input 1 (A1)and A2 and see the results.

As you see the another Excel picture I posted, I measured P1 and P3 at the beginning.

P1 (A1) results are stable but P3 (A2) are not.

I assumed the fluctuation at P3 caused from some noise.

Then, I connected A1 to P2 point. All of sudden the signal became stable for both inputs.

If I do not connect anything to A1, A1 follows A2 values.

If I connected A1 back to P1 points, then A2 became unstable again.

If I connect A1 to some other voltage than, A2 follows that value, not measuring Vpp of P3 point any more.

Why does that happen?

Is this normal?

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You talk about an amp with a gain of 10, followed by a buffer (no schematic diagram).
But in your other post, I see a diagram with a gain of 69.
?

Maybe better to continue this in your original post.

http://forum.arduino.cc/index.php?topic=338872.0

3

Wawa:
You talk about an amp with a gain of 10, followed by a buffer (no schematic diagram).
But in your other post, I see a diagram with a gain of 69.
?

Maybe better to continue this in your original post.

How to take care of noise for Peak-to-Peak detection of sine wave?? - Sensors - Arduino Forum

Sorry one of the picture was not uploaded because of the size.

This test is not from my post before, it's separate one.

I am wondering why analog input stability is affected by the another analog input data.

4

There is only one A/D for all analogue inputs.
A MUX (multiplexer) is used to switch between analogue inputs.
A "ghost charge" could still be present from a previous read.
The output impedance from the driving circuit has to be low enough to clear that quickly.
The output of an opamp should be low enough.
Taking two consecutive readings, and only using the second one, is another way to tackle the problem.

Reading can also be influenced by Arduino's 5volt rail if you are using default Aref.

Did you check the output of the circuit with a scope.
Leo..

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