Why Do Some IEPE Sensors Specify “Excitation Voltage”?Hardware: Modular Instruments>>Dynamic Signal Acquisition and Analysis (DSA)
Problem: IEPE or ICP devices universally require constant current excitation (i.e. 2 to 20mA) for power. Why do some sensors also specify Excitation Voltage and does it have anything to do with the Compliance Voltage specification for National Instruments’ IEPE products? Solution: The short answer is that a sensor’s Excitation Voltage specification must be less than or equal to the Compliance Voltage specification to utilize the full measurement range of the sensor. NI’s use of 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 [1] the signal produced by the sensor, [2] the bias voltage produced by the sensor, and [3] any common-mode voltage as seen by the input channel. (Note: This common-mode voltage excludes the voltage drop across the 50ohm resistor caused by sinking the excitation current.) As an example, the PXI-446x Compliance Voltage specification is worded in this way.  A sensor manufacturer’s use of Excitation Voltage is another way of saying “Your excitation circuitry should be able to drive a constant current through at least this much voltage difference.” The total voltage difference across an IEPE sensor’s terminals is generally a sum of output bias voltage and the output signal. As an example, here is a specification excerpt for PCB accelerometer model 352C03.  As you can see the output bias voltage could be as high as 12V and the output signal could be as high as 5V. The sensor may have a voltage drop as high as 17V across its terminals so it specifies a minimum Excitation Voltage of 18V. *Note: The output bias voltage is a function of the IC used in each sensor and not a function of excitation current. An excitation current of 2mA incurs as much bias voltage as an excitation current of 10mA. **Note: The 30VDC 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. So the next question becomes, what if the sensor’s Excitation Voltage specification exceeds the NI device’s compliance voltage specification? This may or may not be a problem depending on the signal level you are trying to measure. If you are measuring signals across the full scale range of the sensor, then your signal may experience clipping before it saturates the input channel of the measurement device. If you are measuring small scale signals however, then as long as the output bias voltage plus the peak signal voltage plus any common-mode voltage (typically very small common-mode noise) 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. PCB has written an article describing the “Effect of Excitation Voltage on the Dynamic Range of ICP Sensors.” (see Related Links) An example of a sensor/device conflict would be the NI 9234 with the PCB triaxial accelerometer model number 356A32. Out of the 19V of available compliance voltage, this accelerometer could use up to 17V just in output bias voltage. This leaves less than 2V of input range available for accurate measurements. Related Links: PCB Piezotronics: Signal Conditioning Basics for ICP and Charge Output Accelerometers PCB Piezotronics: Model 356A32 Products and Services: NI 446X Specifications Attachments:
Report Date: 04/14/2009 Last Updated: 09/11/2009 Document ID: 4WDE2H7E |
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