I wonder if anyone has seen this before and has ideas on how to fix or troubleshoot?
I have a set up with an ethernet shield (this one: http://www.oomlout.co.uk/ethernet-shield-for-arduino-updated-p-188.html) and a TMP36 temperature sensor. I'm trying to report the temperature remotely over ethernet. I'm using the A5 analog input having read elsewhere that the A0 and A1 inputs have pullup resistors attached in the ethernet shield.
When powered from USB on my laptop, everything works as expected - I see pretty stable temperature values from second to second.
However, when powered from an external 9V adaptor [this one: http://www.oomlout.co.uk/wall-power-adapter-9v-600-ma-p-236.html], the analog input becomes extremely noisy when read with analogRead on the arduino.
I've tried a few debugging steps, here's what I found:
I have metered the Vcc and signal levels with a multimeter in both scenarios and they are both very stable, including under load when I make an HTTP request to the device. I also tried printing out a value read 500ms in the past in case the act of serving an HTTP request introduced the noise, but this shows just as much noise in the externally powered setup.
Has anyone seen this before or able to suggest the next step in debugging?