4.2. IEPE Input
IEPE stands for "Integrated Electronics Piezo Electric". It has been established as the industrial
standard for piezoelectric transducers. Other brand names for the same principle are ICP
®
,
Isotron
®
, Deltatron
®
, Piezotron
®
etc. The integrated sensor circuit transforms the charge signal
of the piezo-ceramics, with its very high impedance and high EMI sensitivity, into a voltage sig -
nal with low impedance. The converted signal can be more easily transmitted. At this signal or -
dinary low cost coaxial cable with a length of more than one hundred meters can be used.
A peculiarity of IEPE is that power supply and measuring signal use the same line. So, an
IEPE
transducer needs, like a transducer with charge output, only one single-ended cable. Figure 4
shows the block diagram. To separate the low impedance sensor signal from the power supply,
the integrated circuit is supplied with constant current. This constant current must be fed into
the measuring line and simultaneously separated from the subsequent amplifier stages.
The flow of constant current into the sensor it is indicated by the permanently lit white LED
“IEPE”.
By supplying the sensor with constant current a positive DC voltage arises over its terminals.
This bias voltage depends on manufacturer and specimen and is usually between 5 and 15 V.
The sensor signal is superposed on this bias voltage. The output voltage of the transducer never
changes to negative values. Its minimum value is the saturation voltage of the integrated imped-
ance converter (0.5 V to 1 V). The supply voltage of the constant current source determines the
maximum value of the output voltage. For the M72 this voltage amounts to 24 V and guarantees
an optimum dynamic range for all available sensors. Figure 5 shows these relations.
7
Figure 4: IEPE Principle
Integrated amplifier
U
I
const
s
Q
U
Piezo
ceramics
C
C
R
I
C
C
I
const
R
I
Coupling capacitor
Constant supply current
Input resistance
U
s
Supply voltage of
constant current source
coaxial cable,
> 100 m
IEPE Transducer
Signal Conditioner