sinusoidal(noise) function in the output of a photodiode transimpidance circuit, DC output expected.

Thread Starter


Joined Nov 27, 2019
Hi everyone!

This is my first post, I've been working in Photodiode transimpidance circuit as a color sensor, my problem is that the output is not DC as I expected, but is sinusoidal(120Hz) function I think this is noise but I do not know what is the source or how to mitigate it.
Any suggestion would be really appreciated!



Joined Apr 5, 2008

Is the photo sensor shielded for surrounding light?
It looks like that a 120 Hz ( 2 X 60 Hz from the mains) light is giving trouble.


Thread Starter


Joined Nov 27, 2019
Thanks Bertus and MrChips for repply,

1-Sensor is not shielded for surrounding light (Lab lamps incident on it)
2- I will test it batteries to rule out problems from AC mains


Joined Aug 21, 2008
The mains voltage can get into the circuit through modulating the light in your lab and by capacitive coupling to the input of your transimpedance amplifier.

Those long, unshielded leads make great capactitive pickups, those leads should not be used because they pick up interfering signals and the added capacitance to ground will increase the noise out of the preamp. The sensor needs to be on the same assembly as the preamp (the amplifier with the 5.6 Meg Ohm feedback resistor) and the whole thing shielded from the mains. Been there.
Top of my head, Florescent lights. Waveform would help.

I don;t think florescent light illumination waveform is sinusoidal, but I think it's 120 Hz based on some UV measuremnts I made that used a neon light transformer. The current builds up, the arc strikes and then the current is limited by the reactor. Since it's bipolar, the frequency would be 2x the line frequency.

Incandescent probably would be the same.


Joined Oct 2, 2009
Been there, seen it. I had a student with the same problem. Turned off the room lights and the problem went away.
(Of course, turning off the lights is not the solution. I just demonstrates the symptom.)
How I ran into the problem was building a quick and dirty UV intensity meter, The custom 4x 4" lamp (500 to 1000 W) used a neon light transformer. The detector was a calibrated UV detector for 254 nm.

They were not getting the right answers. I'm not sure if we checked it against a photolighography meter. So I cooked up an I-V converter run of 9V batteries and said you had to use this Fluke TRMS meter. That worked fine. There were very few TRMS meters around the lab.

I scoped the output and it was really strange, but made sense.

Thread Starter


Joined Nov 27, 2019
Hi guys,

Thank you very much for your suggestions. Finally the problem was easy solve. The 120Hz as you guys told me, was due to the fluorescent lamps. I just made and enclosure for the photodiode and the 120Hz signal disappeared.