Measurement Computing ZonicBook 618E rev.3.4 - Manual (Page 67)

Functional Concepts of the Circuitry
AC / DC Input Coupling
Channels TACH1 through TACH4 each have an input coupling feature. Input coupling can be set to “AC”
or “DC” coupling. After the tachometer input signal leaves the differential amplifier, DC coupling [if
selected] sends the signal directly to a high speed comparator. AC coupling, which is typically used, sends
the signal through a 1 Hz high pass filter before it reaches the comparator.
AC coupling is typically used as it rejects DC offsets, which are of no practical use to tachometer
applications.
DC coupling rejects nothing and using it sends both AC and DC components to the comparator.
Input Coupling
The input coupling stage, shown in the preceding figure, is compatible with tachometers that have high and
low voltage outputs within the voltage range of –75V to +75V. A wide range of input waveforms can be
accommodated since the ZonicBook/618E’s comparator threshold can be set anywhere from –12.5V to
+12.5V.
Many times the best way to accommodate unbalanced inputs is to AC couple and set the threshold at
0.00V. This forces the comparator to trigger on the center of the waveform, where the input has high slew
and little noise.
Although ZonicBook/618E’s circuitry has a wide input dynamic range and good common-mode rejection,
you should connect the tachometer GND to the Analog Common, when possible.
ZonicBook/618E 917695 Tach Channels 7-3