Measurement Computing CIO-EXP-GP - Manual (Page 32)

7.4.3 Selecting the Excitation Voltage
10
V +5
C
O
MP
X2
.5 GN
D
0.
5V
1V 2V 4V
S17
RE
M
X1
DIP switch S17 has five switches to select bridge
excitation voltage. Only set one ON. All others must
be OFF.
Figure 7-2 shows the switch and the excitation power
source jumper set for the factory defaults. Excitation
is set for 10V ON. Power source must be set to +12V
(as shown in Figure 7-1) or +PEXT.
Do not select an excitation voltage at the switch that
exceeds the excitation power supply voltage.
Figure 7-2. Excitation Voltage Select Switches
7.5 Determining the Appropriate Gain
To accurately measure a voltage, the full scale of the signal should be matched to the full range of the
input circuit. (Most DAS boards have an input range of ±5V, which is the native range of the analog to
digital converter at the heart of the board. Some DAS boards include amplification on the input circuit to
allow the signal to be amplified to make better use of the resolution of the A/D.) For example, an input
signal which varies between 0 and 1 volt would only be using 1/10th of a ±5V A/D converter's
resolution. By switching the input signal of the DAS board to unipolar (no negative voltage) and
amplifying the sign wave signal by 5, the entire range of the A/D converter is used and a higher
resolution measurement may be made. By adding this gain and selecting this range, the resolution on a
12-bit A/D improves from 2.4 millivolts per bit to 0.24 millivolts per bit. If you needed to measure a
change of 1 millivolt, you would need an amplification of 10.
In order to match your signals with the input range of the A/D board, you should do a similar calculation
and set switches on the CIO-EXP-GP for the required gain. Remember to make sure that the settings in
InstaCal match the switches on the DAS and CIO-EXP-GP boards.
When using strain gauges, the expected output from the sensor should be calculated and the gain of the
CIO-EXP-GP set accordingly. There are some examples at the end of this chapter detailing these
calculations. You may also find it helpful to refer to the Appendix for additional strain gauge
information.
7.6 Setting the Gain
Once you have determined the gain required for your application, set the gain of the CIO-EXP-GP using
the following guide.
Amplification for ALL channels (board output gain) is switch selectable (S17) for X1 or X2.5.
Input amplification for EACH CHANNEL is switch selectable (CH0 through CH7) for X1, X10, X100 or
X1000. A user-specified gain may be set by supplying a precision resistor at position RX### and setting
the “U” option on switch CH ## to ON.
28