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Excitation and Compliance Voltage for IEPE Sensors in NI Cards

Updated Sep 2, 2022

Reported In

Hardware

  • PXIe-4464
  • PCI-4461
  • PXI-4461
  • PCI-4462
  • PXI-4462
  • NI-9233
  • NI-9234
  • NI-9230
  • NI-9231
  • PXIe-4492
  • NI-9250
  • NI-9218
  • NI-9232

Issue Details

Why do some IEPE and ICP sensors specify Excitation Voltage, and how does that relate to the Compliance Voltage specification for National Instruments’ IEPE products like the NI 4461, NI 4462, NI 4464, NI 4492, NI 9230, NI 9233, or NI 9234? Is my IEPE sensor compatible with my module?

Solution

In short, the Excitation Voltage for the sensor must be less than or equal to the Compliance Voltage for the measurement device to ensure that the full measurement range of the sensor can be utilized.

Excitation Voltage

Excitation Voltage is the voltage level required by a sensor.  This is applied at a constant current.  The total voltage difference across an IEPE sensor’s terminals is the sum of output bias voltage and the output signal. As an example, here is a specification excerpt for PCB accelerometer model 352C03:
 

 
The excitation needed for this sensor is dictated by the following: 
 
Excitation Maximum = [Highest Measurement Range Value] * Sensitivity + Maximum Bias Voltage
Excitation Minimum = [Lowest Measurement Range Value] * Sensitivity + Minimum Bias Voltage

Thus:
 
Excitation Maximum = [500g pk] * 10mV/g + 12V = 17 V and
Excitation Minimum =  [-500g pk] * 10mV/g + 7V = 2 V

Based on the Excitation Maximum and Minimum values we calculated, we need a module that has a compliance voltage maximum of 17 V and that the range includes at least 2 V. 
 

Compliance Voltage

Compliance Voltage is another way of saying “What is the largest voltage drop that the IEPE circuitry can handle while maintaining its constant current supply?” The combined voltage drop across the IEPE circuitry is a sum of:
  • The signal produced by the sensor
  • The bias voltage produced by the sensor
  • Common-mode voltage as seen by the input channel excluding the voltage drop across the 50 Ω resistor caused by sinking the excitation current (typically very small common-mode noise)
As an example, the PXI-4461/4462 Compliance Voltage specification is worded in this way: 
 

Using requirements we calculated earlier, 2V and 17V, and the datasheet above, we can determine that the PCB accelerometer model 352C03 would be compatible with the PXI-4461/4462 since both of those values are within the Compliance Voltage range.

Additional Information

What if my sensor’s Excitation Voltage exceeds the Compliance Voltage specification? 
If you are measuring signals across the full scale range of the sensor, then your signal may experience clipping. If you are measuring smaller magnitude signals, and the output bias voltage plus the peak signal voltage plus any common-mode voltage is less than the device’s Compliance Voltage specification, then you are operating within the acceptable bounds for both the sensor and the measurement device. 

The 30 VDC upper end specification for “excitation voltage” is carryover terminology for users trying to select an external current supply. For our use case we are interested in meeting the minimum excitation voltage specification.