GEN7iB
250
INPUT CARDS
INPUT CARDS
12
Sense lines
Remote sensing of excitation voltage, is commonly recommended for use with preci-
sion, commercial transducers to prevent lead-wire resistance changes (due to changes
in either temperature or length) from affecting transducer span, or sensitivity. Lead-
wireattenuationpresentsapotentiallysignicanterrorsourceintransducersutilizing
a Wheatstone bridge circuit. The lead-wires represent a parasitic resistance, and a
portion of the excitation voltage intended for the bridge circuit is dropped in the lead-
wire system, reducing the voltage actually present at the transducer, and effectively
reducing the transducer sensitivity.
Bridge balance
The bridge circuit is only in balance (has zero output when the bridge voltage is
applied) when R1 / R2 = R4 / R3. Taking the various resistance tolerances on the strain
gauge(s), resistors and lead wires into account, an initial unbalance is invariably pres-
ent. Adjusting the initial balance so that there is zero output at zero strain is achieved
by bridge balancing.
Shunt calibration
Eachbridgeamplierchannelcontains100KΩ,0.1%xedprecisionresistorthatcan
be switched in by software control. With a gage factor of 2.00, this resistor simulates
thefollowingvaluesofdeectionforvariousbridgecongurations.
Deectionforvariousbridgecongurations
100KΩ
BRIDGE 1000Ω 350Ω 120Ω
mV/V
2.4888 0.873 0.299
μstrfullbridge
1244 437 150
μstr½bridge
2488 873 300
μstr¼bridge
4975 1747 600
A second calibration resistor can be connected to the connector pin externally. Either
one of the shunt calibration resistors can be switched in by software control to provide
multi-pointcalibrationandlinearityverication.
Shielding and immunity increase
UsinghighbandwidthampliersliketheGN840B/GN1640B,anyexternaldisturbance
typicallyisimmediatelyreectedinthemeasuredsignal.Tominimizeexternaldistur-
bance pick-up the excitation, sense and signal leads are generally separately twisted
and shielded within the cable to minimize the cross-coupling that would otherwise
occur.
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