NI 6210 oscillations

[hellboy123]

I'm using the NI DAQ 6210 analog inputs to analyse the data coming in from a photodiode amplifier, bandlimited from 0.1Hz to 500Hz using appropriate filters. I'm using the Differential input mode to acquire data (AI4 as AI + and AI 12 as AI -). As I'm using a charging laptop (I think the chassis is grounded) and a regulated power supply derived from 240V to power the opamps, I tried connecting the outputs in Differential mode with two 10k resistor to bias both inputs to the Ground (Pin 28) in one case, and 200k in another. These are the outputs I got, in the absence of any light in the room (Dark room with metal doors).

Fig. 1 Using 10k as the bias resistor

Fig. 2 Using 200k as the bias resistor

With the laptop charger unplugged I was getting this.

When I checked the same signal on a CRO, it was giving only a few mV of noise (insignificant). So i'm sure this is not the kind of signal that I'm expecting. Possibly, this is the problem with the ground loop between the DAQ board and the power supply. Any ideas on how to remove these oscillations?

[johnsold]

The spectra show strong components at 50 Hz and harmonics, but the large high frequency peaks may not be at harmonics of 50 Hz.  The signal with the 200 k bias resistors appears to be closer to 454 Hz based on the time domain graph.

Did you check the signal with the oscilloscope while the DAQ devices were connected?  Does connecting the scope change the data collected through the DAQ?

I think you have some kind of connection problem and that a ground loop is probably part of it.  However, there may be other factors as well.  Can you post a scheatic of your complete wiring connections?

Lynn