Battery powered sensor connected to myRIO

[kumarasova]

I am using myRIO's analog input channels (Connector C: AI0+ and AI0-) to read Thorlabs photodetector (DET36A/M) which is battery powered and its voltage output is current based. I am using my phone's flashlight to test the photodetector. 

The measured voltage kept going to -10 V in between the measurements (figure 1). 

After I found that the output of the photodetector is current based,. I used a buffer amplifier (figure 2) in between the photodetector and myRIO's analog inputs. Now the output is offset to zero, but the measured voltage still oscillates between -0.06 to .05 V.

I used an oscilloscope to cross verify. It is able to measure the output with or without the buffer amplifier.

Can anyone please suggest what I can do?

[alyssa.h]

Hey kumarasova,

Would it be possible to explain more about what your expected output should look like? From your post, I realize that you are getting what you want, but you never fully described what it should actually look like (pictures of what the oscilloscope sees would be helpful to compare to).

Regards,

Alyssa H.

National Instruments

[kumarasova]

Hey Alyssa,

I figured out what the problem was. In the myRIO, I had to connected Agnd to AI0- . The signal looks similar to what I read in the Oscope (figure 1).

Do you know why I have to do this?

Because, connecting the Agnd to AI0- spoils the purpose of differential measurement, I am essentially doing single ended measurement now.

[kumarasova]

The ripples (in figure 1) caused by the buffer amplifier is different issue though.