User`s manual

XP8500 I/O Configurations s 27
Selecting Gain and Bias Resistors
The section How to Set Up An XP8500 provided representative values
of gain and bias resistors for the XP8500s conditioned channels. This
section provides a detailed explanation on how to calculate these values
for a particular range of input voltages. Figure 3-8 shows a schematic
representation of the signal conditioning for channels CH0CH3.
Figure 3-8. Schematic of XP8500 Signal Conditioning
Step 1. Select Gain Resistor
The gain and bias resistors, R1R8 (R
g
and R
bias
in Figure 3-8), determine
the input signals voltage relative to ground, as well as its range. For
example, assume the XP8500 must handle an input signal spanning -5 V to
+5 V. First select gain resistor R1 to suit a voltage range of 10 V.
The gain of the amplifier is the ratio of its maximum output-voltage swing
to the swing in the software applications maximum input voltage. The
2.5V input range of the TLC2543 A/D converter chip (U3) limits the
LMC660 (U2) op-amps output swings to 2.5 V. Therefore. Equation (3-1)
expresses an amplifiers gain in terms of the range of its input voltage.
where g is the gain, V
IN
max
is the maximum input voltage, and V
IN
min
is the
minimum input voltage.
The ratio of the user-specified gain resistor R
g
(R
g
= R1, R3, R5, or R7) to
its associated fixed input resistor (RP4A, RP4C, RP3A, or RP3C) deter-
mines an amplifiers gain. Equation (3-2) provides the gain for the
configuration shown in Figure 3-8 with the input resistor fixed at 10 kΩ.
–
+
CH0–CH3
VREF+
A/D
converter
+5ANA
10 kΩ
10 kΩ
U3
U2
VREF+
+5ANA
RP2
+5ANA
0.01 µF
RP3
RP4
RP3
RP4
R
g
R bias
VRn-
VRn+
AIN0AIN3
minmax
ININ
VV
V2.5
g
−
=
(3-1)
Ω
=
000,10
R
g
g
(3-2)