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.