USB-4431 vs NI9209, noise on signal input

[stinf]

Hello,

I have a strange noise problem on USB-4431, not present, with the same setup, on NI9209 mounted on a cDAQ-9174.

Setup:

- pressure sensor, 0-10V output

- 3-wire cable from pressure sensor: power wire, output signal wire, ground wire

- the power and ground wires are connected to a bench power supply

- the output signal and ground wires are connected to the voltage input of USB-4431 or NI9209; the cable is 4 meters long on NI9209 and about 1.5 meters on USB-4431 (with the coaxial section about 30 cm long)

NI9209 is configured with single-ended inputs, USB-4431 only allows pseudo-differential inputs. Both devices are powered by the very same PC through an USB connection.

By using Ni9209 I see no 50 Hz noise, and peak-to-peak noise is about 1.5 mV.

With USB-4431 (which should be a more accurate device) I see about 10 mV of peak-to-peak noise and a relevant 50 Hz noise. Noise is present even when I only connect the cable, with no sensor and no power supply. This fact led me to think there is some issue on my cable, but building a new one had no effect.

I need to use USB-4431 as the system with NI9209 is used elsewhere.

Please give some help on how to solve the issue.

Thanks in advance

Stefano

[ZYOng]

There are a few suggestions. Since the noise is 50Hz, it could be a powerline noise. Try Eliminating Powerline Noise from DC Measurements in NI Software - NI and AC and DC Coupling for NI DAQ Devices.

Another thing to point out is that single-ended and differential connections have different wiring requirements. Since you are measuring a pressure sensor which is a floating signal source, you might need to add bias resistors. See Field Wiring and Noise Considerations for Analog Signals.

[stinf]

Hi ZYOng, thanks for your suggestions. I will try them as soon as possible and reply the results here.

Stefano