3
2. Function and Operation
2.
1. Introduction
Fig. 2 shows the block diagram of Model M68 with its
most important functional groups.
U
Q/10
Q
U
Q/10
Q
Q1
Q2
ICP
supply
J1 (ICPon/off)
Low pass
Frequency
High
pass
0.1Hz
3Hz
V2
1. Int.
2. Int.
V1
Gain
a
v
d
Gain
Input
Overload
Output
ICP
+
Compa-
rator
>5%
V=1
Compa-
rator
Fig. 2: Block diagram
Description
of the
Signal Path
Depending on the position of the input switch, the input
signal passes the impedance converter Q1 or Q2. If ICP
operation is selected, the signal is directly connected to
the amplifier. At ICP
operation a constant current is fed
into the input socket to supply the sensor electronics.
The constant current source can be switched off by the
internal jumper J1, in case an AC voltage shall be con-
nected to the input.
The input circuit is followed by the first amplifier stage,
low pass and high pass filters. The low pass filter has 6
selectable limiting frequencies. The high pass filter has
a limiting frequency of 3 Hz, which can be bypassed by
the switch “HIGH PASS / INTEGRATOR”. In this case
the full bandwidth down to 0.1 Hz comes into effect.
The filters are followed by the second amplifier stage.
The divided gain before and after filtering provides
sufficient dynamic range, even for signal components
outside the filter range. At the same time a high signal-
to-noise ratio is achieved.
Before reaching the output driver the signal may pass
one or two integrating stages. The output is DC cou-
pled.
A control LED for the output modulation indicates an
output signal higher than 5 % of full-scale modulation.
An overload LED shows if the output signal exceeds
