User`s manual

XP850028 s I/O Configurations
Given a range of 10 V for the input voltage, Equation (E-1) fixes the
amplifiers gain at 0.25. This gain correctly scales the input signals range
to the op-amps 2.5 V maximum output range. Therefore, R
g
must be
2500Ω.
Step 2. Calculate Bias Resistance
Next, if the op-amp is to servo its output properly around the desired
center voltage, the appropriate bias voltage needs to be established at the
op-amps noninverting input. Select the bias resistor, R
bias
, to position the
input-voltage range correctly with respect to groundin this example, -
5V to +5V.
The value for R
g
has already been selected, and so the maximum input
voltage, V
IN
max
, determines the maximum voltage seen at the amplifiers
summing junction (inverting input)circuit nodes VR0 to VR4.
Compute VR0 to VR4 using Equation (3-3).
The bias voltage, V
bias
, must equal its corresponding VRn for each op-
amp. A voltage divider, which consists of a bias resistor, R
bias
(R
bias
= R2,
R4, R6, or R8), and a fixed 10 kΩ resistor (RP4B, RP4D, RP3B or RP3D),
derive this bias voltage, V
bias
(V
bias
= VR0+, VR1+, VR2+, or VR3+), from
VREF+. Note that VREF+ is not necessarily the same as REF+. (REF+ is
the positive reference voltage the A/D converter chip uses.)
The XP8500s conversion mode determines which reference voltage the
op-amps uses. When the XP8500 operates in the absolute mode, VREF+
is 2.5 V and R
bias
is
When the XP8500 operates in the ratiometric mode, VREF+ is +5 V, and
Continuing the example, the gain is 0.25 and V
IN
max
= +5 V; V
bias
is then
1.0V using Equation (3-3) R
bias
, therefore, is 6667 Ω in the absolute mode
and 2500 Ω in the ratiometric mode.
Step 3. Choose Best Standard Resistor Values
The calculated resistor values, of course, will not always be available. In
these cases, use the nearest standard resistor value. For example, use
6650Ω (1% resistors) instead of 6667 Ω, or use 6800 Ω (5% resistors).








+
×=−
g1
g
VVR
max
IN
n
(E-3)
.
V-V
000,10V
R
bias
bias
bias
2.5
Ω×
=
(3-4a)
.
V-V0.
000,10V
R
bias
bias
bias
5
Ω×
=
(3-4b)