VTI Instruments EMX-4016B, User Manual

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VTI Instruments Corp.
APPENDIX B 70
Figure 4-23: AC Coupling Voltage Group Delay Frequency Response
IEPE I
NPUT
F
AULT
D
ETECTION
IEPE input fault detection is accomplished in hardware by monitoring the DC bias of the input
signal. IEPE sensors have a DC bias between 9 V dc and 15 V dc when they receive a DC current
(usually between 4 mA dc and 10 mA dc). The AC signal riding on the DC bias is the signal
proportional to the physical measurement. An open or short condition at the input is detected by
monitoring the DC bias voltage at the input. A SHORT is declared when the DC bias is less than
threshold voltage VTL and an OPEN is declared when the DC bias is greater than threshold voltage
VTH. It is possible for the unit to declare a failure if the AC signal is low frequency (<0.5 Hz) and
its peak amplitude (DC+AC) exceeds the threshold voltages VTL or VTH. The table below shows
the typical threshold voltages VTL and VTH for each Model.
Model VTL VTH
EMX-4250/EMX-4251 1.30 V 21 V
EMX-4350/EMX-4380 0.95 V 20 V
Table 4-6: Typical VTL and VTH Threshold Voltages
The front panel fault LED in the EMX-4350 and EMX-4380 will turn RED if the input is either
OPEN or SHORT and the module is set to IEPE and the fault can also be read through software.
The EMX-4250 & EMX-4251 can control the fault LED in the EMX-4016 16-channel break-out-
box (BOB). This is done through the I2C serial communication lines available in the 25-pin input
connector.
TEDS
Each channel has the ability to read and write TEDS data from IEEE 1451.4 TEDS (Transducer
Electronic Data Sheet) enabled transducers. The figure below shows a connection diagram of an
IEPE sensor with a 1-wire memory containing TEDS information. Schottky Diode D1 is used to
block the IEPE current powering the IEPE sensor from damaging the memory chip. D1 needs to
have a low (<0.5 V) forward voltage. Note that the memory chip is connected backwards: its ground
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
1E-1
1E+0
1E+1
1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5
Gr
ou
p
D
ela
y
(s
ec)
Frequency (Hz)
EMX-4350 EMX-4380 EMX-4250, 1kpC-10kpC EMX-4250,100pC-500pC