Noise on analog input when using an ethernet shield

Hi everyone,

I use an arduino pro mini and i read A0 for values from a sct-013 30A/1V Non-invasive AC Current Sensor Clamp.

When i have the ethershield disconnected i read 0 values as i should (no ac power connected).

However when i connect the ethernetshield i immediately get values like 34-37 instead of 0 (possible analog values 0-1024).

I tried to place various capacitors between V5+ and ground and between A0 and ground without significant change. I powered the arduino from USB/PC and USB/power adapter without change. I also tried another ethernet shield but i didn't noticed anything different.

Can anyone suggest something to reduce the noise?


Can you give link to the current clamp and to a schematic that you use ?

This is the current transformer :

It is noise ? Perhaps the current from the Ethernet shield causes a change of a ground level. The ground currents are very important. Can you make a photo of your wiring ? Did you somehow use AREF or analogReference(), or is everything with the 5V reference ? What are the values of the resistors to get 2.5V ? Show us the sketch please.

Hi Koepel,
Thank you for your responce.

The clamp is identical to the one on your link. 30 A correspond to 1V read via A0.


  1. clamp
    arduino A0 to clamp signal
    arduino GROUND to clamp signal
  2. ethershield
    arduino D10 to SS
    arduino D11 to MOSI
    arduino D12 to MISO
    arduino D13 to SCK
    arduino V5+ to V5+
    arduino GROUND to GROUND
  3. powered via USB

About ref:
No i use analogReference(INTERNAL); //initialize referance to 1.1 V in the schetch
I haven’t connected any resistors, the clap has internal resistors and brings 0-1Volts corresponding to 0-30Amps.

oops! small correction..arduino GROUND to clamp GROUND!

As i said before I tried to power the arduino from different sources than the USB. I tried once more with a more reliable power adapter and not through the USB of the arduino. It worked this way. Now I get 0 volts in analog port when i am connected to the ethernet!