8.2. High Pass Filter
The M72 Signal Conditioners have a high pass filter with a lower limiting frequency of 3 Hz
(-3dB). This eliminates low frequency noise which may otherwise occur with piezoelectric com-
pression accelerometers under the influence of temperature transients (see Chapter 3.3.2). The
second-order Butterworth filter has a slope of 40 dB / decade. The 3 Hz high pass is switched on
and off by pressing the “HP/Integrator” button. When the high pass is switched on the green
LED “3 Hz” lights up. When the integrators are in use the high pass is always active.
If the high pass filter is switched off the minimal lower limit frequency of the amplifier, 0.1 Hz
(-3 dB) is available.
The frequency response is shown in Figure 6.
8.3. Integrators
The M72 Signal Conditioners can measure non-integrated, single and double-integration sensor
signals. Integration is particularly useful for vibration measurement with accelerometers on ro-
tating machinery. Single integration of vibration acceleration generates velocity, whereas double
integration generates displacement.
The “HP/Integrator” button activates the integrators. Single integration is indicated by the or-
ange lit LED “Vel.” (velocity) and double integration by the orange lit LED “Disp.” (displace-
ment). If there are no integrators in use then both lights remain switched off.
When the integrators are switched on the 3 Hz high pass filter is always activated.
The crossing point of the of the frequency response graph with and without integrators is
15.92 Hz (see Figure 6)
The following calculations show how the M72 output voltage u
out
relates to the three vibration
quantities, whereby the M72 configured voltage gain G
u
in mV/mV and the accelerometers sen-
sitivity B
ua
(see data sheet) is:
Vibration acceleration a
(without integration):
a
=
u
out
G
u
⋅
B
ua
(a in m/s²; u
out
in mV; G
u
in mV/mV; B
ua
in mV/ms
-2
)
Vibration velocity v
(single integration):
v
=
u
out
G
u
⋅
B
ua
⋅
10
(v in mm/s; u
out
in mV; G
u
in mV/mV; B
ua
in mV/ms
-2
)
Vibration displacement d
(double integration):
d
=
u
out
G
u
⋅
B
ua
⋅
100
(d in µm; u
out
in mV; G
u
in mV/mV; B
ua
in mV/ms
-2
)
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