VTI Instruments EMX-4016B, Руководство пользователя

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APPENDIX B 65
IEPE
IEPE sensors operate by receiving a DC current (usually 4.5 mA) to power the internal electronics.
A properly operating IEPE sensor will establish a DC bias voltage of 9 V dc to 15 V dc at its output.
The AC voltage output is proportional to the physical measurement measured by the transducer.
The voltage output from IEPE accelerometers can be converted back to Engineering Units (g’s or
m/sec
2
) by dividing the measured volts by the sensitivity (mV/g or mV/m/sec
2
) of the IEPE
accelerometer. Note that other types of sensors besides accelerometer can be of IEPE type. There
are IEPE microphones (mV/Pa) and IEPE force sensors (mV/lbf).
IEPE sensors are known by other registered trademark names depending on the manufacturer: ICP®
(PCB Piezotronics), Isotron® (Endevco), Piezotron® (Kistler), DeltaTron® (Brüel & Kjær).
The IEPE current source needs to be set to the desired amplitude, usually > 4 mA and input coupling
needs to be set to AC. It has a very high compliance voltage (>21 V) and high output impedance.
The current source is polarized and must be operated with positive current flowing from the (+)
terminal to the (-) terminal. The IEPE current source can be enabled individually per channel.
Channels that do not have a current source enabled can be used for standard voltage measurements.
The EMX-4250’s input is set to pseudo-differential when set to IEPE. The LO side of the input is
shunted to amplifier signal ground through a 100 Ω resistor.
Each channel’s current source in the EMX-4350 and EMX-4380 is isolated, allowing operation of
all channels in IEPE mode to float relative to one another and maintaining the input of each channel
as fully differential.
The frequency response of the IEPE sensor is affected by the cable capacitance, the amplitude of
the IEPE current source, and the maximum voltage swing. The following equation can be used to
calculate the approximate maximum frequency:
F
max
=
I−I
s
C∗L∗V∗K
•
I: amplitude of IEPE current source (amps)
•
I
S
: current required by the IEPE sensor to power its internal electronics (usually around
0.001 A)
•
C: cable capacitance per foot (usually between 15 pF/ft to 30 pF/ft)
•
L: cable length (ft) L can be in meters if C is given in pF/m
•
V: maximum output peak voltage swing from sensor (volts)
•
K: factor depending on amplitude accuracy: K = 2.9 for -0.5 dB; K=4.5 for -0.1 dB
Figure 4-6 below shows the frequency response for C = 20 pF, V = 5 V pk, K = 2.9 (-0.5 dB),
I
S
= 1 mA, I = 4.5 mA or 10 mA.