Peekel Instruments User manual PICAS V2.6.1
Page 7 of 59
2 Connecting the instrument
2.1 Carrier frequency inputs on CA2CF
The following pages show examples of the various options of how to connect various input
signals and transducers to the instrument. Later in this document, further details are given
of how to actually measure these signals.
2.1.1 Connecting the Transducers
The carrier-frequency amplifier is mainly used for straingauges and lvdt’s. They are
connected in full-, half- or quarter-Wheatstonebridge configurations, having 4, 2 or 1
external straingauges, resistors, inductances or capacities respectively. The other arms of
the bridge can be completed with the internal, on-board, ½- and ¼-bridge complementary-
resistors. (As a standard, these are 240 Ω for 1/2 bridge and 120 Ω for 1/4 bridge.)
The precise value for a half-bridge completion is not important as long as these resistors
are stable and in balance. The value of a quarter-bridge completion resistor, however,
should fairly accurately match the external straingauge, otherwise a too large unbalance
(offset) will be the result.
All drawings show dotted lines, connecting the ±
SE
with the ±
EX
lines. These are the sense-
lines and must be connected, even when not 6 but only 4 wires to the straingauge-bridge
are used.
The drawings include polarity-signs within the straingauge-resistors. These indicate the
polarity of the amplifier-output-signal for increasing strain and increasing resistance.
It is strongly recommended to use shielded cables.
2.1.2 About Cable-capacitance
A topic, inherent with the use of CF-amplifiers (contrary to DC-amplifiers) is cable-
capacitance. The capacitance between cables to a straingauge-bridge yields a parasitic
impedance, parallel to the arms of the Wheatstone bridge. Any unbalance in capacitance
may therefore lead to errors in the measured signal.
This becomes crucial in quarter-bridge configurations, where the capacitance comes
directly across one arm of the bridge.
(Example: every 1 meter cabling of 100 pF/meter, connecting a 120 Ω bridge to a 5 kHz
carrier-frequency amplifier, gives rise to 100 µV/V C-signal offset. The carrier-frequency-
amplifier luckily does suppress this C-signal by at least a factor 1000. However, this works
only if the amplifier is not overloaded by the C-signal. The C-signal therefore should not be
more than 4...7 times the selected measurement-range of the amplifier. In the most-
sensitive range of 100 µV/V this would allow for 10 meters of cabling.)
The presence of such a large C-signal is not recommended though. In quarter-bridge
configurations therefore, it is common practice to compensate the capacitance by a fixed
capacitor, built in the other arm (between pins +
EX
and ¼).
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