3.3.2. Low-Frequency Noise
When the M72 is used with high gain and with the lower frequency limit of 0.1 Hz (high pass
off) it may be sensitive to temperature transients. Even draft may result in output voltage fluctu-
ation of some hundred millivolts at frequencies around 0.1 Hz. To reach thermal balance a
warm-up time of some minutes is recommended before measurement. For measurements out-
doors it may be advantageous to isolate the device by foam rubber or similar materials.
Piezoelectric sensors have the so-called pyroelectric effect which may cause similar distur-
bances. This phenomenon is particularly strong with compression type and bending type (beam)
transducers. Here it is also important to maintain stable temperatures.
If lowest frequencies are not of interest the 3 Hz high pass should always be used. With the inte -
grators (see Chapter 8.3) the 3 Hz high pass is always on.
4. Inputs
The Signal Conditioners M72 are designed for both sensors with charge output and sensors with
integrated impedance converters to IEPE standards. Both input types use the same BNC input
socket. Select the input type by pressing the “Input” button repeatedly. The current status is in-
dicated by a blue LED for charge input or white for IEPE. If neither of the LEDs are lit, the in -
strument is running as a voltage amplifier without IEPE power supply.
4.1. Charge Input
Capacitive signal sources, usually piezoelectric sensors with charge output, are connected to the
charge input. The input stage is a capacitive feedback amplifier
,
which transforms the sensor
charge signal into a voltage.
The ratio of output charge (U) to input charge (Q) is determined by
the capacitance of the feedback amplifier (C).
U
=
Q
C
The M72 Signal Conditioners have two charge converter stages. At the lowest gain range a
charge converter with a gain of 0.1 mV/pC is used. For the other gain ranges a charge converter
with 1mV/pC is active.
To ensure that the output charge reverts to zero after a specified period of time without a charge
signal, discharge resistors are located over the feedback capacities. The resistors are designed in
such a way that a lower frequency limit of 0.1 Hz is obtained. This makes manual reset no
longer necessary.
The advantage of charge measurement is that cable capacitance and insulation resistance have
almost no influence on the measuring results. For sensors with charge output it is strongly rec -
ommended to use special low-noise cables. Ordinary cable will cause a considerable measuring
error under mechanical stress, as a result of the so-called triboelectric effect. Cables with low in-
sulation resistance, for example caused by humid connectors, reduce the accuracy of measure -
ment at lower frequencies. A desirable insulation resistance is higher than 10 G
W
. Cables longer
than 10 m are not recommended at the charge input. Proper shielding is also important.
6