ADC noise on A1 but not in A0?


I'm interfacing Arduino Duemilanove with 2 sensors, LCD and computer.

Problem is values read from A1 fluctuate: I'm getting values between 450 and 550 when my DMM sees a very stable 2.50 V. It is working with the default reference, so I would expect to see fluctuations about 0.25 V above/under 2.50 V on the DMM, but this is not the case.

Most shocking is the other sensor does not see noise: it fluctuates only by 1 or 2 values. DMM measurement is very stable here, too.

So I guess I'm picking some noise. After googleing the forum:

  • I powered Arduino from a wallmart. No luck.

  • Both sensors outputs are op-amp outputs, so I guess the low impedance needed by Arduino ADC is OK

-I put 0.1uF bypass caps on all op-amps and Arduino's AREF.

  • Wires from the sensors to Arduino are less than 6 inches long.

  • Tried to change A1 to A5 with no luck. Putting A1 on A0 and A0 in A1 does not work, but then the other sensor begins to fluctuate, too. After undoing this change, sensor on A0 fluctuates until Arduino is restarted...

So I'm getting out of ideas. What could I do?

TIA, Roman

Wallmart sucks dont power your arduino with them :-)

Try to tell us what the sensors are and how you hooked them up, that will give us something to go with.

Thank you, it seems that the problem is sensor-related.

Putting A1 on A0 and A0 in A1 does not work

Sorry, this was to mean "does not solve the problem"

But we can't go any farther for lack of details. Back to you.

Yes, of course. I'm back to the breadboard to search for faults. Since the problem indeed seems not to be arduino-related, I fear that posting it here would be off-topic. I don't want waste your time, so going to extensive debugging first seems to be the right thing to do.

Thank you very much for your help