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...

[Henrik_Volkers]

Just saw, that the DAQ has no BNC input, so I assume you use the shown adaptor close to the DAQ.

So use short cables from the adaptor to the DAQ. 

Use differntial inputs AI1(17) to adaptor + (pin) and AI9(18) to adaptor - (shield) 

additional a resistor from adaptor - to AIGND(28) , resitor value 1M, 1k, 100 Ohm or just cable.

If your shielded cable is long or you later want to measure higher frequencies  , add an additional 50 Ohm ( 2x 100 Ohm) at the screw terminals of the adaptor .

Other things, that migth fight the ground loop:  use the same powerline for the sensor power supply and the computer that provide the USB port for the DAQ.  (I don't know US(?) power distribution systems that well, but if you have more than a 1 phase system and you happen to use a different phase, you can get higher ground loop currents/ common mode voltages due to build in powerline filters)

[jdemar98]

First off thanks for responding!

I purchased this cable: https://www.thorlabs.com/thorproduct.cfm?partnumber=CA2012

I should be in the lab in the next few hours to do some testing. 

I am fairly certain the error is due to EMC, the sensor itself has been tested capped, with multiple power supplies and light sources.  Everything is on the same power line. The pc powering the daq, the photodiode and the laser.

I read that part of the manual " 50 Ohm output and shielded cable", but i think i misinterpreted it. Haha.

can i use ai5 for + and ai13 for shield/- ? Also, can I put the 100Ohm resistor between ai13 and AI GND ? I am no longer using that adapter. 

Basically, AI5 +, AI13-, 50 ohm between AI5 and AI13, 100 ohm between AI13 and GND.

Once again thank you.