Why Bias Resistor is Required in Differential and RSE Mode

Updated Jun 24, 2020

Reported In

Hardware

  • Multifunction I/O Device

Driver

  • NI-DAQmx

Issue Details

Why do differential and NRSE mode require bias resistance?

Solution

Each electronic instrument has an insolation category that can be measured reliably up to a specific voltage. For NI devices, they are classified as insolation level categories 1 to 4. Most NI hardware has an insolation rating of Category 1, and Category 2 or 3 if it is higher. The criterion for this insolation rating is how much element can withstand the magnitude of the earth potential reference voltage. Category 1 refers to a potential within 100 V, and Category 2 refers to a potential range near 220 V, an AC commercial power source. The higher the insolation rating, the less the insolation breaks, so you can safely proceed with the measurements.

 

 

In terminal configurations of DAQ device, Differential and NRSE modes require bias resistor(R). See this document for detailed configuration instructions. (Difference Between Differential, RSE, and NRSE Terminal Configurations )

  • In the case of RSE, it doesn't matter with bias resistors because it refers to the signal ground of the measurement system.
  • In the case of Differential or NRSE, they need bias resistors because they do not refer to the ground within the measurement system but to the ground of the signal source.
Because we don't know what potential they have. Of course, there is no problem with sensors or signal sources that correspond to most categories 1. but if there is any possibility, potential values that exceed the measurement device's insolation rating may be entered. Doing so will cause damage to the device lead to inaccurate measurements. So, measurement system will require adjustment of that potential. There are reason why bias resistor requires.
  1. Bias resistance adjust the DC voltage level of the signal.
  2. Bias resistance remove offset due to input bias current from the PGIA.

Additional Information

Refer to this document for detailed size configuration of the bias resistor. (How to Select Bias Resistors)