Bias resistors in grounded system

[NikOett]

We have a number of USB-6218 dataloggers, packaged into 19" racks with BNC inputs/outputs. These are used for a range of purposes, but primarily tend to get used for logging the outputs of pressure transducers and force balances. These are configured to accept 16 channels of differential inputs.

In the lab, these devices are powered through grounded power supplies and therefore (I assume, although please correct me if I am wrong) should be connected as depicted in Table 1 (differential, grounded signal source) of the Field Wiring and Noise Considerations for Analog Signals document.

However, I would also like to use the same datalogger when taking data in a car, with the 6218 powered though the USB connection of a laptop and the pressure transducers powered from the vehicles 12V supply. This would indicate a non-grounded configuration and therefore I should install a bias resistor connecting AI(-) to AI GND. As an aside, when using a laptop it seems that there will be no ground for the datalogger?

If I would like to use this datalogger for both grounded and non-grounded applications is there a way in which I can use the same configuration? For instance, what would be the effect if I use a bias resistor when measuring a grounded signal source?

Many thanks 

[david.bolt]

Dear NikOett

Laptop computers performing measurements can create a floating situation similar to using a floating source. Because a laptop computer does not have a common ground, you may see floating signals even if your source is grounded. In this situation, instead of the signal source floating, your measurement hardware itself is at a floating voltage.

Even though you connect your laptop power supply to a wall outlet, this problem continues because most power adapters only provide a differential voltage and do not actually connect the laptop to ground. Depending on the way the laptop is built, connecting the laptop's ethernet or modem cable sometimes provides this common ground and stabilizes your system.

Some methods for providing a common ground are the following:

• Use the Reference Single-Ended (RSE) by connecting a grounding pin from your signal source to the AIGND pin on your DAQ device.
• If using a differential configuration, use bias resistors (usually between 10k-ohm and 100k-ohm) to connect the positive terminal to AIGND and the negative terminal to AIGND as you would with a floating source.
• If using a Non-Referenced Single Ended (NRSE) configuration, use bias resistors (usually between 10k-ohm and 100k-ohm) to connect the positive terminal to AIGND and the AISENSE to AIGND as you would with a floating source.

[NikOett]

Dear David,

Many thanks for your reply. Since I'm using the DAQ in a differential configuration with DC coupled signals, I'm going to follow the route of connecting bias resistors between the AI- and AIGND terminals.

I am still a little confused by some apparent inconsistency in bias resistor advice though - in the user manual for the 621x series here, Figure 4-24 seems to show a configuration for the USB-6215/6216/6218 in differential mode without bias resistors (but still a connection between the AI- and AIGND terminals), whereas Table 4.2 for USB-6210/6211/6212 shows the use of bias resistors. I'm wondering why there should be a difference and could you confirm that the use of bias resistors is appropriate for the 6218?

Many thanks,

Nicholas