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Monitoring ICP® Sensor Bias

Benefits of Checking ICP Sensor Operation

ICP sensors require a constant current power source providing 2 to 20 mA at 18 to 28V DC. In most applications, 2 to 4 mA current is adequate. In special applications driving long cables (>100ft) at high frequencies (100k Hz), higher current is required to avoid high frequency attenuation. For specifics, please view the driving ICP sensors over long cables reference nomograph.

Do not apply a DC voltage directly to an ICP sensor with either a power supply or a voltmeter. Applying DC voltage directly to the ICP sensor, without current limiting, can damage the sensor’s internal electronics.

Unlike high impedance charge mode sensors, checking the impedance across the connector pin to ground of an ICP sensor with a dmm will not provide meaningful information. Consult the information above to avoid damage to the ICP sensor’s built-in electronics.

To check operation of an ICP sensor, connect the sensor to a PCB power/signal conditioner that has a circuit integrity meter monitor which can display sensor bias voltage or perhaps bias LED indicators on newer products.

When an ICP sensor is connected to a PCB constant current power unit through a good cable, the meter will indicate normal sensor bias voltage of 9 to 13V (Mid Green), or around 3 to 8V (Lower Green) for some low bias sensors (seismic, cryogenic and certain low noise models), or 14 to 17V (High Green) for sensors with built-in circuit gain.

DMM and Color coded meter readings for various sensor bias voltages and cable connections are indicated below:

Sensor BiasDMM ReadingColor Coded Analog Meter
Most ICP Sensors9 to 13V"GREEN" (Mid Scale)
Low Bias ICP Sensors3 to 8V"GREEN" (Low End)
High Bias ICP Sensors14 to 17V"GREEN" (High End)
Cable open circuit*24 to 28V"YELLOW" (Full Scale)
Shorted cable*0 Volts"RED" (Low End)

*Install new cable (or remove old cable and check end-to-end continuity of center conductor and shield with a DMM). Be sure when connecting the cable to the sensor to turn the cable’s floating connector and do not turn the sensor. Turning the sensor may cause shavings off the center pin causing shorts or alternatively the cable connector pin to catch and also rotate tearing free of its wiring connection and cause an “open” in the circuit.

Function of Color Coded PWR Unit Circuit Check-Out Meter

Reasons Why an ICP Sensor May Not Turn On

  • The ICP sensor is not connected to a constant current Power Unit. Inadvertently, you may have a charge mode sensor connected to an ICP power unit or an ICP sensor connected to a charge amplifier (other than some dual mode models which do have an ICP input).
  • The Power Unit is not turned on (signal will go through, but highly attenuated).
  • Using incorrect power as ICP sensors require +18 to +28V DC at 2 to 20 mA constant current. Consult your manufacturer for appropriate power and current for a given model.
  • If a battery power unit is being used, its supply voltage may have dropped below the sensor's bias voltage. Check “Battery Test.” (The positive voltage swing of an ICP sensor is from the sensor’s bias voltage to the supply voltage. The negative voltage swing is from the sensor’s bias voltage down to about 2 volts.) Remember, a low supply voltage limits the sensors positive going voltage swing and dynamic operating range.
  • An ICP sensor with a high bias is being used with a supply voltage that is too low. Some special ICP sensors with gain may have bias voltages of 15V or higher. When connected to a 15V 2 to 4 mA power unit, the sensor will not turn on. This was a common problem with early FFT analyzers that incorporated a 15V 2mA “ICP Input."
  • Some ICP sensors with extended low frequency response (i.e. long discharge time constants) and voltage overload protection may take longer to turn on, e.g. seismic accelerometers, high range pressure and force sensors. After applying power, wait a few minutes to see if the sensor turns on. Most modern dynamic sensors are hermetically sealed through laser weld cases and glass sealed connectors. If an older or non-hermetically sealed sensor is exhibiting trouble with bias turn on (for example if the sensor has not been used for a long time), it is possible to heat soak it overnight in an oven at 225° F (or less if the sensor is rated to a lower temperature limit), then recheck for bias turn-on.
  • Microelectronics in the sensor could have been damaged by applying voltage directly to the input; too much measure and exceeding the 100V max input to the IC; or possibly by ESD.