I'm having a bit of trouble removing some noise from my circuit coming (somehow) from the mains power.
The device I've designed is portable and to be used outside, so I don't expect the noise it to be an issue with the final design but for lab-based testing I'd still like to eliminate it.
The circuit is just a basic TIA/photodiode measuring a LED light (modulated slowly at 0.2Hz) which is driven from the Arduino. The light levels are very low - such that I'm getting a few hundreds of picoamps of photocurrent. I figure that this is where the noise is getting coupled into the system (this photocurrent is driven through a gain resistor of 100Mohms). I've tried a few things to remove it, but it is proving to be quite elusive. Looking at the signal with the CRO I don't see any modulation at 0.2Hz, but instead a larger 50Hz peak every time the LED is on, which then decreases in size when the LED switches off. This doesn't make sense to me - I would have thought I'd see a DC increase in the level with the LED on and the 50Hz noise being the same regardless if the LED is on or off.
The lasted attempt involving putting the whole setup (Arduino, TIA circuit & batteries) within a diecast aluminium box, and run the photodiode through coax. I grounded the circuit to the metal case, such that the anode of the photodiode (via the coax) shares this ground.
Is this the best bet? Next step will involve taking the whole setup to a complete new area to see if is location specific.
Any suggestions would be much appreciated.