Noise considerations using a 6034E Card

[gopinath]

I am measuring an analog signal in the differential mode using a 6034E Card. I am using a CB-68 pin connector block (unshielded) and a shielded cable (SH1006868). The signal I am measuring is 1 Volt and I get noise levels (peak to peak) of roughly 100 millivolts (single point - not averaged) when measuring with LabVIEW. I verified this with an oscilloscope and observed the same value for the noise measured.

However, when I disconnect the cable from the connector block, with the signal still connected to the oscilloscope (and the terminal block), I notice that the noise levels measured on the oscilloscope drop to 60-70 milli Volts.

Has anyone else observed this? Is this an expected behavior? How is it explained?

If not, sh
ould I believe that the actual noise level on my signal is only 60-70 milli volts and it is the measurement setup (card/cable?) that is adding the noise?

Any help with this will be greatly appreciated.

Thanks.

[JoshuaP]

Hey Gopinath,

A common reason for additional noise when connecting a sensor to a DAQ device is normally caused by grounding loops. If you have a grounded signal and it is being wire to the DAQ in a RSE fashion, it will have two grounds and create a grounding loop. We recommend you connect your device and take your measurements using differential mode.

However, if you are connecting a floating signal (one that is not referenced to ground) to the DAQ device and you are using NRSE or differential mode this can also cause problems. However, in the case of floating signals with no reference to ground in the system all you need to do is add an additional 100kOhm resistor between your reference signal (Ch-) and AIGND.

In addition, I would recommend using some s
ignal conditioning like a SCXI or SCC to setup some low pass filters to filter out the 60-70 mV noise you always see. In most cases the noise will be 60Hz noise caused by lights, motors, and power supplies in the proximity.

Here are a few links to help you out.
http://digital.ni.com/public.nsf/websearch/1F72E5DC795EFB85862562980052BC13?OpenDocument
http://digital.ni.com/public.nsf/websearch/9D66EDB25F8F533D862562980056605E?OpenDocument
http://digital.ni.com/public.nsf/websearch/632470D5285E52328625629800537049?OpenDocument
http://digital.ni.com/public.nsf/websearch/1FFCBE3451ABEE09862562970059881D?OpenDocument
http://digital.natinst.com/public.nsf/websearch/D509679FFAE2764386256297005D0C9D

I hope this helps out.

Joshua P.
Application Engineering
National Instruments