VTI Instruments Corp.
APPENDIX B
66
Figure 4-6 –Frequency Response of IEPE Sensor Affected by Cable Length
Charge (EMX-4380)
The charge amplifier is used to convert the charge output (in picocoulombs - pC) from Piezo-Electric
(PE) transducers to volts. IEPE sensors are PE transducers with built-in charge amplifiers. PE
sensors are used in high temperature environments where the built-in electronics cannot survive or
operate poorly such as inside jet engines. The charge amplifier does not have DC response. The
charge amplifier operates from an isolated power supply, so the BNC input shell is isolated from
chassis ground.
The charge amplifier is tested by using a series capacitor Cin (see Figure 4-7) to convert volts to
pC. A 1000 pF capacitor converts a 1 V pk sine-wave into 1000 pC pk sine wave. The gain of the
charge amplifier is 1 mV/pC nominal (±2% over temperature), so the output of the charge amplifier
produces 1 Vpk sine wave output. Accuracy of the charge amplifier gain calibration is dependent
on how accurately Cin is known.
Figure 4-7: Voltage to Charge Conversion by Using Series Capacitor
The EMX-4380 can operate with input isolation turned ON or OFF. When isolation is ON, the input
LO side is floating from the digitizer ground and the voltage output of the charge amplifier is
injected into a differential input voltage amplifier. This makes the input look like a differential
charge amplifier since all common mode signals are passed through the charge amplifier and
rejected by the differential input voltage amplifier located in the main PCB. Both differential and
single-ended charge output sensors can be connected to the EMX-4380 when isolation is ON. When
0
50
100
150
200
250
300
350
100
200
300
400
500
600
Fr
e
q
u
e
n
cy
(k
H
z)
Cable Length (ft)
4.5mA,5V,0.5dB
10mA,5V,0.5dB
Cin
Vin
Qin = Vin * Cin
Csh
Vout
PE Sensor Model
Cable
Capacitance
