6
2.
3. Inputs
The Signal Conditioners M68 are designed for both
sensors with charge output and with integrated imped-
ance converters to ICP
®
standard as well. You can
switch from one to the other type of transducer by
means of the slide switch next to the input socket. Both
types use the same BNC input socket.
Charge
Mode
Capacitive signal sources, usually piezoelectric sensors
with charge output, are connected to the charge input
(Q). The input is fed to an amplifier with capacitive
feedback. All M68 instruments have two input stages
for charge. In the position “Q/10” of the switch the gain
is divided by 10.
The advantage of charge measurement is, that cable
capacitance and insulation resistance have almost no
influence to the measuring result. For sensors with
charge output it is strongly recommended to use special
low-noise cables. Ordinary cable will cause a consider-
able measuring error at mechanical stress, as a result of
the so-called triboelectrical effect. Cables with low
insulation resistance, for example caused by humid
connectors, reduce the accuracy of measurement at
lower frequencies. A desirable insulation resistance is
higher than 10 G
Ω
. Cables longer than 10 m are not
recommended at the charge input.
ICP
Mode
The abbreviation ICP means “Integrated Circuit Pie-
zoelectric”. It has been established between many other
names as industrial standard for piezoelectric trans-
ducers. The integrated sensor circuit transforms the
charge signal of the piezo-ceramics, with its very high
impedance and high EMI sensitivity, into a voltage
signal with low impedance. The converted signal can be
easier transmitted. The cable length at this input may be
more than one hundred meters. Ordinary low cost co-
axial cable can be used.
A peculiarity of ICP
®
is, that power supply and meas-
uring signal use the same line. So, an ICP
®
transducer
needs, like a transducer with charge output, only one
single-ended line.
Fig. 4 shows the circuit diagram. To separate the low
impedance sensor signal from the power supply, the
integrated circuit is supplied with constant current.
