NI USB 6009 pegged at 316mV when connected to a BPW34 photodiode

[rodrigosantos]

I have an USB 6009 connected to a BPW34 photodiode but no matter what I try, I can only get a constant reading of about 300mV at AI0. When I disconnect the photodiode from the board, it behaves normally at the oscilloscope, but once connected to the board, the reading becomes constant. I tried the standard solutions of putting a resistor between AI0+ and ground but it did not work. I also checked the AI0 connection by feeding a sine wave and it does work. When I connect the AI0 to the oscilloscope without the photodiode, I get a strange reading of about 1.2 V. What could be the cause of those problems? 

[Bob_Schor]

Are you using Single-ended or Differential inputs to the USB-6009?  I strongly recommend using Differential ("high" into AI0, "low" into AI4).

Bob Schor

[rodrigosantos]

I am using Single-ended. 

[Bob_Schor]

Go Differential!  You may have a Ground problem ...

Bob Schor

[rodrigosantos]

Going differential still gives me the strange 1.4V reading and the board still won't read the photodiode. 

[Bob_Schor]

OK, my lack of engineering training is showing -- can you include the schematic you used for the Photodiode, including how you powered it and where you connected the USB-6009?

Bob Schor

[rodrigosantos]

I took everything off the board, there is just the ground of the photodiode on the GND and the signal on AI0. No external power supply for the photodiode. The reading is still stuck at 300mV. 

[RTSLVU]

---

@rodrigosantos wrote:

I took everything off the board, there is just the ground of the photodiode on the GND and the signal on AI0. No external power supply for the photodiode. The reading is still stuck at 300mV. 

---

What do you expect to see? 

The input impedance of the 6009 is only 144K ohm your oscilloscope input impedance is most likely in the 10 megohm range. http://www.ni.com/pdf/manuals/375296c.pdf

I think that is your issue here.