Unexpected Voltage with USB-6211 and Thorlabs Amplified Photodiode

[jdemar98]

Hello everyone,

I'm encountering a baffling issue with my DAQ setup that I'm hoping someone can help diagnose.

My Hardware:

• DAQ: NI USB-6211

• Sensor: Thorlabs PDA100A2 Amplified Photodiode

• Source: Thorlabs PL252 (4.6 mW, 639 nm CW Laser)

I am trying to measure the stable DC voltage from the photodiode. Based on the laser power and the detector's specifications (0 dB gain), I calculate that I should be measuring a stable ~ +3.0V.

When I configure a task for a continuous read (1k Hz) in RSE mode, I'm getting a wildly incorrect, fluctuating reading between -1V and -4V.

When I try a differential measurement (AI0+ and AI8), I get a different, but still incorrect, result, which is shown below.

Also, i'm using one of the below screw terminal adapters to convert the end of the bnc cable to wires to connect to the DAQ. Is there a better way? 

Any ideas on why this is happening and how I should go about capturing a stable voltage readout from the amplified photodiode. would be greatly appreciated.

Thanks.

[Henrik_Volkers]

What you see is your power line frequency,  I counted 3 periodes in 0.05 s resulting in 60 Hz 🙂 

Possible sources i your (not shown setup) are many...

- Sensor:

 - - Extenal (room) light modulated with your power line  (but usually 2 times the line frequency (sin²(60Hz) ) hitting your  senor, test by covering it)

 - - The amplifier of your sensor ( power supply of your sensor). (not my 1 guess ,  

- Your light source, not sending a clean CW  (no, I don't think so, but you can test with a battery powered LED, just cell, resistor, LED) 

- EMC: (my guess) : Ground loop:   But still the value seems quite high...  so cabling , grounding/shielding 

The manual of the senor (RTFM) states a 50 Ohm output  and shielded cable .. 

If you use that connector/adaptor: 

use a tightly twisted cable pair from sensor to your DAQ.

add a 50 Ohm termination at your differntial (!) DAQ input (two 100 Ohm resistors form a 50 Ohm termination 😉 )

add another ground wire from the sensor out ground/shield  to AIGND  (maybe adding another 100 Ohm  inbetween)

Have a look into the DAQ manual about settling time error versus source impedance if you use more than one channel...