Using two NI USB 6351 DAQ boards in parallel for analog input data acquire

[Mr_Engineer]

1. I want to acquire analog  input voltage data with two NI USB 6351 DAQ boards.

2. I'm using chanel no. 0 for both USB 6351 DAQ boards in differential configuration for analog input.

3. I have a single floating voltage signal source ( AC coupled with 70 ohm output impedence ) whose output I'm feeding  into chanel no. 0 of both USB 6351 DAQ boards in paralle.

My qustion is:  Should I need to provide bias return for analog inputs of both DAQ boards  by connecting the negative lead of a floating source to AI GND pin (either directly or through a bias resistor) for both USB 6351 DAQ boards or only for single device? ( Refere to X series user manual for NI 632x/634x/635x/636x/637x Devices, page no.4-11).

[JakeA]

Hello Jamal,

Based on what you've described I would recommend providing bias return for both devices, but I don't think you will need to do this to obtain the measurements you're looking for. You may want to have a look at this white paper, it's a pretty good summary of all of the grounding considerations you may need to make: https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...

Best regards,

Jake

[Mr_Engineer]

Hello Jake,

I'm using single voltage source as analog input for both DAQ devices, so if I ground negative inputs of both DAQ devices I think it will create ground loop. What do you think in this case?