Some resistor modules, such as 40-262 RTD simulator module, have a facility to bias the variable resistor on a common mode voltage. This provides an explanation using 40-262 as an example.
The 40-262 provides an extremely precise variable resistor for simulating temperature variable resistors where the application
requires a very fine resolution. The fine resolution requirement requires the use of two types of variable resistor,
a coarse control which provides a broad range of resistor values and a fine control which is based on using high voltage
digital potentiometers implemented on a CMOS process.
The coarse control is fully isolated since relays provide an isolation barrier between the contacts and the control mechanism (a relay coil), however the fine control imposes some different requirements.
The fine control CMOS potentiometers require the use of a power supply. The power supply is common to all the fine controls on the different channels in the module, it is an isolated design so the voltage on the common is not fixed. That voltage has to be connected to a voltage that then determines the operating voltage range of the digital potentiometer.
If the common mode voltage is set to zero volts (the default setting) then the user connection can be at any voltage from -15V to +15V without creating a problem in the digital potentiometer.. However some applications may for example only need positive voltages on the user connections (for example when the system uses constant current loops to measure the resistor) and may require a voltage capability beyond +15V. This can be achieved by setting the common to a different voltage, for example if connected to PXI +12V the resistor module can have up to +27V applied.
To give users flexibility the 40-262 permits the use of the +12V, -12V or the use of an external voltage reference