NI 9205 to acqiure the voltage from photodiode, it's very unstable

[Bo Ma]

Hi everyone, I used NI 9205 to acquire the voltage signal from pre-amplified silicon photodiode. But the voltage signal is very unstable, looks like period signal, the variable range is about 30mv, which is much much biger than specification. But when i connected to DC power supply directly, it's very stable. I don't know the reason, anf how to handle this issue? urgent. Thanks

[Bo Ma]

PS: when i used the multimeter to measure the output of photodiode, the signal on the multimeter was also very stable. It's very strange.Thanks

[Henrik_Volkers]

What bandwidth do you need?

How fast are you sampling?

Where is your aliasing filter frequency?

What is the bandwidth of your PD amp?

Do you have a fast scope (to borrow) to take a look at the amp output? Maybe it's oscillating ;-o

Your multimeter is very slow and mostly they use integrating sampling methods and/or have appropriate aliasing filter ......

[GarPar]

Hi Bo Ma-

     It is very possible that the voltage from the photodiode is oscillating.  Is it by chance oscillating at a 60Hz frequency?  Electricity is broadcast at 60Hz in the U.S.

     Do you have the photodiode hooked up as a differential measurement?

[Bo Ma]

Thank you for your response. When i connected the photodiode to multimeter, i measured AC signal present, but the DC part was very stable. the signal of the input module connected to Si photodiode looks like sine wave, typically AC signal. I don't know where is the AC signal come from. I also tried differential measurement, it's does not work. i don't know how to handle it. Thanks

[Henrik_Volkers]

By using the Testpanel you can take a look at the sampled signals. Try the highest possible samplingrate to sample about 120ms signal.

You will see ( i suppose) all  kind of noise an hum and somewhere buried maybe the signal you what to see. BTW what do you want to measure with that PD? 

and How do you connect it to your DAQ?

Have you thought about a transimpedance amplifier?