Noise when connecting 6211 USB daq

[ivoc]

Hello!

I'm using NI-6211 multifunctional DAQ to measure signals from a photodiode. But there is a problem - noisy signal. The average dispersion of noise is about 6 mV. In order to check the noise of the photodiode itself I connected it to a Tektronix digital oscilloscope TDS2002. In this case the noise is much smaller - about 1 mV. But when I connect in parallel the ni-6211 adc the signal becomes noisy. The ni-6211 in on but no aquisition takes place.

I attach two figures:

fig1, yellow curve - signal from a photodiode registered by a TDS2002 scope (100 MOhm, DC)

fig2, yellow curve - signal from a PD registered by a TDS2002 scope when NI-6211 is connected in parallel

The input is differential, both the ni-6211 and photodiode are grounded.

Is it possible to reduce this noise.

Thank you very much.

[Henrik_Volkers]

Noise output from the DAQ input.... avoid groundloops ,  sometimes a choke help:

Best solution would be a battery driven TIA (Transimpedanceamplifier) see Fig 7 here (you might choose a lower feedback resistor for lower gain) or search the web for a 5V single supply solution, request a sample OP and heat the solder iron 😉

[ivoc]

Hello, Henrik!

Thank you very much for your rapid answer. Really, the problem was in ground loops. I found out that the photodiode had the isolated inputs. But its "-" signal was connected with ground via the oscillosope input. I solved the problem connecting the AIGRND input to the AI8 ("-") input of the adc. In this case the noise became much less.

At the attachement there are fig1 and fig2 - waveforms registered by a scope without (fig1) and with (fig2) ni-6211 in parallel. The signals are almost the same.

If I connect AIGRND to ground the noise still present. Later I will try to play with bias resistors and grounding. I will write here if it give any positive results.

fig1 (without ADC in parallel):      fig2 (with ADC in parallel):

[alazarev]

Hi ivoc,

Have you had a chance to make any more progress on eliminating the ground loops? I am facing the same problem in our product and trying to gather experience from a community of people who have been successful before.

Many thanks,

Alex