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Photo diode background noise

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alzomor

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Hi I am using a simple reverse biased photo diode with two stage amplifier

The problem is that when the IR source is close to the photo diode this circuit is working fine,
it amplifies the received signal in addition to a small 50HZ signal is super imposed on it

but when the IR source moves away from the photo diode the signal gets smaller but it's super imposed on 50HZ signal,and the 50HZ signal amplitude increases as I move away.

I don't know if the problem from the reverse biasing resistor with the photo diode or from the amplifier ?

Is there any recommendations to get ride of the 50Hz Signal.

Regards
Hossam Alzomor IR.jpg
 

If 50 HZ does not fall into the bandwidth you are interested in, use a 50 HZ notch filter. First off all shield the detector from ALL light. This will kill any 50 HZ from lightbulbs, but not from your wiring picking up the mains magnetic field. This should give you two possible areas to attack.
Frank
 

Possible sources of 50 Hz:

* Flicker from lamps powered from mains AC. (Our eyes do not necessarily detect the flickering.)

* Your power supply lacks adequate filtering, and lets through ripple from mains AC.

* Your input leads pick up ambient mains hum from nearby house wiring and apppliances.
 

@Chuckey
R151 & C150 work as high pass filter with cut off frequency 338.9 HZ which will block the 50HZ

- - - Updated - - -

my problem is not the 50Hz noise itself
My problem is that the 50Hz noise amplitude increases when my signal is weak .
Why the 50 Hz noise increases when my IR Signal is week?
 

I think the IR diode circuit has more sensitivity to interference when it has weak signals because with a strong signal then the diode conducts more current which causes it to have reduced impedance which shorts the interference.
Is the circuit built on a solderless breadboard where all the messy jumper wires are all antennas that pickup mains hum? Then build it on a compact pcb.
Is the photo diode connected with long unshielded wires that are antennas that pickup mains hum? Then use a shielded audio cable.
 

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