NI9215 grounded differential or single ended signal measurement

[f_janasz]

Hello there,

I have a slight problem with my NI9215 - just to describe a situation, I'm measuring 4 channels of low voltage signal (lower than 100mV, mostly in range of 10-50mV) simultanously. At the moment I'm using scan engine to obtain values, will move to FPGA later to increase the acquisition rate. My signal is preconditioned with an amplifier, applying a gain of 1000 to bring it to mV range rather than uV. I'm using well shielded cables. I am not entirely clear whether my signal is grounded differential, or single ended - if I follow this manual:

https://www.ni.com/docs/en-US/bundle/ni-9215-getting-started/page/overview.html

Page 14 most closely resembles my circuit, thus I would assume it to be single ended. However, if I follow this manual:

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/analog-fundamentals/grounding-c...

Figure 8, then in the table the right uppermost option describes my system best. Thus, I've tried two ways of connecting my sensors. First, as shown below in version A:

In this case, the noise levels are not so high, there however exists an offset between all 4 channels, even when measured voltage should be 0. Additionally, I cannot see the signal I am supposed to measure - if I make a secondary measurement with my Keithley 2001 multimeter (connected exactly as NI9215 to the system), I see a clean signal in the range of 10 mV - 40 mV and I can measure none with NI9215.

Then, I've tried the following connection scheme, version B:

The only difference is, I've added the connection between source ground and COM terminal - so to satisfy NI9215 manual, pg 14. In this arrangement, the offset error between channels is gone and I can see the right ampliude of the signal to be measured, but the noise levels are extremely high. I've tried adding a 1MOhm resistor in the line connection to COM, but it had no influence on the signal. I must say I am quite confused on how to resolve this situation. I would appreciate any advice on the matter.

Best regards,

Filip

[farora]

Hi,

the module only supports +/- 10V, better accuracy can be achievef if you choose 9205 with a +/- 200 mV. Furthermore the way how you have to connect the wire depends on the sensor / source. On page - 12 14 you have to choose the right one. https://www.ni.com/docs/en-US/bundle/ni-9215-getting-started/page/overview.html

Is the noise inside the specification of the module?

Best Regards, Fabian

[f_janasz]

Hi Fabian, 

Thank you for your answer, however choosing the right wiring scheme (page 12 or 14) was actually part of my question - I am aware I have to choose the right one, but are unsure which one it should be - also the link to the manual is in my first post already - could you advise how can I make the selection?

At the moment using module 9205 is not possible for me. What I observe right now is that my amplifier circuit is adding a lot of noise to the system. I've moved to FPGA and the noise from shorted NI9215 module is around +/- 1mV. With added low-pass filter with 1kHz cutoff freuquency I can bring it down to +/-0.15 mV.

I was thinking about ungrounding my system, so that I deal with floating signal - that would correspond to figure 5 on page 13 from the NI9215 manual - which suggests to put 1MOhm resistor between minus cable and COM. However if we use the Wiring Considerations manual (https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...  ), the table 1, top row, left column - floating signal, differential measurement, the suggestion is to use two resistors - 10 to 100 kOhm to connect BOTH cables to ground (which I undestand represents COM port in this situation, which is then further connected to earth ground) - it seems to me that those two schemes are incomatible, so which one is the correct one?