Input Conditioning
MEN Mikro Elektronik GmbH
20
3.3
Differential Acquisition (Current/Voltage)
The second standard version supports only 8 channels but has differential inputs.
The input voltage and current rates are the same as for 16 channels (±10V or
20mA). The filter frequency is 3kHz. These rates may also be changed at the
customer's request.
To make proper use of differential acquisition, it is necessary to have basic
knowledge on analog measuring techniques. One common mistake is to believe that
the two wires of a differential input acquisition system can be used like a hand-held
instrument. This is not the case at all. A differential amplifier is an analog
computation circuit. This circuit takes two input voltages referred to ground (!) and
performs analog subtraction.
Unfortunately, in a normal multiplexed acquisition system each input signal has to
pass through a multiplexer. This semiconductor part is only able to handle signals
within the supply voltage range.
The problem is best illustrated by a simple example. Imagine you wish to measure
the voltage and current of a 24V power supply. If you install a resistor divider
between the 24V and ground to obtain a measuring range of 10V, you are not able to
get the voltage across a shunt, because it is outside the working range of the
multiplexer.
Fortunately, the M-Module does not work in this conventional manner. The
M-Module uses one differential amplifier for each channel. Of course, the amplifier
also needs a reference ground for analog calculation but this amplifier has a
common mode range of ±200V. Only the result of the calculation, which is normally
between -10V and +10V, is switched by a multiplexer. This is of great practical
advantage.
As for single-ended acquisition the current measurement version has a 62.5-
Ω
shunt
installed for full scale at ±20mA. The gain factor selected by software must be 8!
Figure 2.
Block diagram of the differential input
The following tables give the standard pin assignments for differential versions.
Filter
Differential
Amplifier
Multiplexer
8
Shunt only for current measurement
