VTI Instruments Corp.
50
EX1629 Basic Operation
M
EASUREMENT
R
ANGE
/
G
AIN
Each EX1629 input channel can be individually configured with respect to its signal conditioning
gain. The differential voltage measurement range of the EX1629 is ±150 mV, ±1.5 V, or ±15 V,
for gain settings of 100, 10, and 1, respectively. As strain measurements will nearly always be
conducted at a gain of 100, its measurement range is primarily a function of bridge configuration
and excitation voltage level. The extent of initial bridge imbalance, which is reflected in the
unstrained voltage measurement, is a secondary factor, but it is normally not large enough to be
significant. Nominal measurement ranges for some common bridge configurations are listed in
Table 3-3.
Bridge Configuration Excitation
Range
Quarter
10 V +30927 µε/-29126 µε
Quarter
5 V +63829 µε/-56603 µε
Half
10 V
±15000 µε
Half
5 V
±30000 µε
Full
5 V
±15000 µε
Full
2.5 V
±30000 µε
T
ABLE
3-3:
EX1629
M
EASUREMENT
R
ANGE
For quarter-bridge configuration, note that the dynamic range is slightly different for tension vs.
compression. While the dynamic range of the voltage measurement circuitry is a balanced
±150 mV, the transfer function of strain-to-voltage is nonlinear, and that results in the small
disparity.
The default gain setting is 1, although most strain gage measurements are taken using a gain of
100.
E
XCITATION
S
OURCE
Each EX1629 input channel features an independent excitation source that is not only
programmable on a per channel basis, but also with respect to the positive and negative supplies
that compose the total excitation voltage. This programming independence provides the flexibility
of balanced or unbalanced excitation. Specifically, the positive and negative excitation voltages
are programmable from 0 V to +8 V and 0 V to -8 V, respectively, with a total current capability
of 50 mA per channel. Overcurrent protection is 60 mA. Moreover, each source is independently
regulated, such that an overcurrent condition on one channel does not affect the regulation of any
other channels.
The operations to program and enable each excitation voltage are discrete. Excitation voltages that
are not enabled output an actual value of 0 V, regardless of their programmed value.
When the excitation source is changed, the nominal value of the total excitation voltage is updated
in the EU strain conversions. However, for highest accuracy, the excitation voltage should be
measured and the measurement used in the EU conversion (see the
section that follows). The strain accuracy tables are based on the requirement that
excitation measurement is performed.
For highest accuracy in half-bridge and full-bridge configurations, each excitation source has a
remote sense connection. In order to properly remove the effects of lead wire resistance, these
lines should be connected at the same point that the ±Excitation lines are connected to the bridge.
The remote sense lines are always active in the circuitry; there is no control to turn on/off remote
sense. Because of this, it is critical that they be left open (unconnected) in quarter-bridge
configuration, where their connection would be invalid.
