ZWorld Exp-A/D12, User Manual

Page: 30 / 111

XP8500 30 s I/O Configurations
Figure 3-9. Effects on A/D Converter Input from Adjusting Resistor Values
Then measure the voltages at VR0- and at the U2 op-amp output. Because
the currents through the input resistor and R
g
are essentially identical, the
ratio of the voltages across the resistors is equivalent to the ratio of the
resistances. Therefore, the gain is
Again using Channel 0 as an example, measure the voltage VREF and the
voltage at VR0+ (see Figure 3-8). Because the current into the op-amp
input is negligible, the resistance ratio of the two resistors in the voltage
divider alone determines VR0+. The value of the fixed resistor in the
divider can then be calculated based on R
bias
and the value of VR0+.
0
10
2.5
XP8500 Input (V)
A/D Converter Input (V)
Op-amp output voltage
deviation arising from
resistor variations
Out of range
Out of range
A/D converter's
input voltage
limit
.
R
R
- VR0
- VR0 V(U2)
g
IN
g
OUT
=
−
=