Spider DSA User’s Manual
211
0.00349/GF. When 120 Ω gages are used, the change is 0.142 Ω, equivalent to a
strain of -0.00119/GF.
Thirdly, the bridge output voltage, V
0
(the difference between In+ and In-), is
read by a high impedance instrumentation amplifier and fed to a 24-bit analog-
to-digital converter (ADC), providing a high precision digital strain time-history.
Fourthly, a nulling voltage, V
Null
= -V
o
, is added to the V
0
bridge output voltage.
This voltage is provided by a 16-bit digital-to-analog converter (DAC). Its purpose
is to force the bridge output to zero upon a computer instruction. That is, it
provides an automatic bridge null upon command.
Fifthly, the bridge excitation voltage, V
in
, (between EXC+ and EXC-) may be
software selected to be ±2.5, ±5 or ±10 VDC. Each of the eight bridges measured
may draw up to 30 mA current from the Spider-80SG.
Sixthly, each channel provides a second ADC to measure the actual V
in
voltage
applied to a bridge (or partial bridge) remote from the Spider inputs. Finally, the
Spider provides a ground (GND) terminal for the shield that noise-protects cables
running to remote strain gages.
Figure xx6: Spider-80SG input channel functional diagram
Connecting Strain Gages
Gages may be interfaced to the Spider-81SG in any of three basic configurations
termed Quarter Bridge, Half Bridge and Full Bridge. By far the most common
(and simplest) installation is the quarter bridge configuration. The half and full
bridge configurations offer definite improvements regarding temperature effects,
sensitivity and linearity. But for most dynamic strain measurements, a quarter
bridge solution is adequate.
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