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Entry  Fri Apr 16 11:27:01 2021, Aidan, Update, 2um Photodiodes, 2um DC photodiode voltage weird drift with no incident power DC_2um_voltage_weirdness.png
    Reply  Fri Apr 16 13:05:05 2021, Aidan, Update, 2um Photodiodes, 2um DC photodiode voltage weird drift with no incident power IMG_2132.jpgFM30_31_w_latter_filtered.pngIMG_2134.jpgbias_bypassed.png
Message ID: 2557     Entry time: Fri Apr 16 13:05:05 2021     In reply to: 2556
Author: Aidan 
Type: Update 
Category: 2um Photodiodes 
Subject: 2um DC photodiode voltage weird drift with no incident power 

We put the preamp output directly into a multimeter and observed the same fluctuating behavior as the DAC channel was changed.

We're bypassing the relay to see if that makes any difference. The old relay wiring (to be bypassed) is shown in the attached diagram. That didn't do anything.

We're looking at filtering the DC output by 5kHz to see if there are any resonances at higher frequencies that might go away. Changing SR560 output for AC path to DC and setting gain to 1 on that unit. Also changing gain in FM31 filter bank from 1E-3 to 1. The results are shown in the attached time series. The channels FM30 and FM31 see the same thing. The only difference is that FM31 goes through an SR560 with a 0.03Hz pole (6dB).

Success by bypassing the DAC bias voltage. We switched to a 300mV bias voltage from a function generator. Doing that removed the causal PD voltage drift induced by changing the laser diode current set voltage (see the last time series). So the issue is some weird coupling into the DAC bias voltage.


[Aidan, Radhika, Nina]

We noticed that the DC channel readout (FM30) of the JPL A1 photodiode is drifting around. What we observe with no light on the photodiode, is the DC output drifiting around. It gets particularly bad when we apply voltage to other DAC channels.

For example, the attached plot shows the DC voltage from the photodiode as I change the set voltage to the laser diode driver. To be absolutely clear, the laser driver itself was completely powered off. I'm just varying the voltage going into the set point BNC connector on the back of it.

For reference, the set up is:

DAC (300mV bias) > relay > PD > relay  > FEMTO preamp (1000x gain) > ADC channel FM30



Attachment 1: IMG_2132.jpg  2.288 MB  Uploaded Fri Apr 16 14:17:35 2021  | Hide | Hide all
Attachment 2: FM30_31_w_latter_filtered.png  16 kB  Uploaded Fri Apr 16 14:48:07 2021  | Hide | Hide all
Attachment 3: IMG_2134.jpg  3.089 MB  Uploaded Fri Apr 16 14:54:19 2021  | Hide | Hide all
Attachment 4: bias_bypassed.png  15 kB  Uploaded Fri Apr 16 15:04:50 2021  | Hide | Hide all
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