differential input to SCXI 1120 with a 1305 input block - noise

[gorgeman]

Hello,

I'm trying to measure low amplitude, DC voltage signals using an SCXI 1120 with the 1305 terminal block. I'm picking up ground noise and I'm having a hard time finding my way through the manuals to solve my issue.

Currently I'm tieing the negative output of my load cell to the ground, as shown in the attached diagram. This is similar to Table 1, row 1, column 1 of the NI wiring document (http://zone.ni.com/devzone/cda/tut/p/id/3344).

Reading the 1120 user manual (http://digital.ni.com/manuals.nsf/websearch/7D3481BCFAA9E300862567EF0071F865), there are two setups for DC inputs in Figs 3-2 and 3-3. My current setup is similar to 3-2, but I have a CMV of zero, so this setup is not required. It seems that the setup in Fig 3-3 would be better for me, but I can't seem to use that configuration with the 1305 terminal block.

Fig 3 of the 1305 user manual (http://digital.ni.com/manuals.nsf/websearch/B02C64965878A95186257449005FBFAC) shows that you can link the negative to ground through a bias resistor, but specifies that for the 1120 this is to be left floating. In fact, in the text above Table 1 of the manual it specifies in italics that this be left in position G.

Is it possible to have the setup shown in Figure 3-3 of the 1120 manual using the 1305 terminal block? Should the setup that I have be okay? When I tie the load cell chassis to ground I get a noise signal that is periodic at 60 Hz (I'm in Canada), but has spikes and plateaus in it, but at low amplitue. If I remove the ground connection I get a relatively clean 60 Hz, sinusoidal noise component with an amplitude of ~0.5mV. My signal is at about 10mV, so this is significant for me.  Using the second configuration I could digitally notch filter at 60 Hz, but I'd rather not.

Thanks for your help.

[WillPwr]

Hello, Gorgeman!

I think you'll find this document useful: Referencing a Floating Input to Ground Using the SCXI-1305

Please let us know if you have any questions!

[gorgeman]

Hi Will,

I will try that. What I take from this is that I can use the "F" position for the ground switch in the 1305, but it will break the isolation of the 1120, which is not bad for me.

I will report back when I have a result.

Thanks!

[WillPwr]

You are correct, gorgeman - I'm glad this sounds like it will work for you.

Let us know how it goes!