Calculating PXIe-4311 Common Mode Error with High Side Sensing

Common mode error is a factor that may offset your PXIe-4311 measurements when measuring with a high side shunt resistor. It can be calculated and compensated for your system. 

The figure below illustrates the recommended wiring when using the TB-4309 ST with the PXIe-4311. See PXIe-4311 Block Diagram for more details.  

PFI_GND is connected to the system ground, and the common-mode voltage source becomes the system’s power rail. The shunt resistor, then, is the differential signal source.

The common mode voltage in this case can introduce a common mode error to your measurements. You can calculate and compensate for this error using the Common Mode Rejection Ratio (CMRR) and the common mode voltage value: 

1. Look up the corresponding PXIe-4311 CMRR.  
   • CMRR will change depending on operating range of the instrument and signal frequency. The table below shows the PXIe-4311's CMRR values. See PXIe-4311 Specifications for more details. 
      
     
2. Multiply the CMRR by the common mode voltage (or your system’s power rail). 

For example, if operating the PXIe-4311 in the -150 to 150mV range with a common mode voltage of 5V and a signal frequency of 10Hz, the common mode error expected would be: 

Common mode error = CMRR*(common mode voltage) 

Common mode error = (1.6uV/V)*(5V) = 8uV 

Please note that the PXIe-4311's self-calibration procedure calibrates the module's internal path, but it does not remove system level common mode error. If a concern, the common mode error can be measured and compensated as described above.