Datasheet

14
Signal Chain Design Guide
The feedback capacitor (Cf) is used for circuit stability.
The device’s wiper resistance (Rw) is ignored for first order
calculations. This is due to it being in series with the op
amp input resistance and the op amp input impedance is
very large.
Circuit Gain Equation
Programmable Gain Amplifier
The MCP6SX2 PGA Thermistor PICtail Demo Board
features the MCP6S22 and MCP6S92 Programmable Gain
Amplifiers (PGA). These devices overcome the non-linear
response of a NTC thermistor, multiplex between two
inputs and provide gain. It demonstrates the possibility of
measuring multiple sensors and reducing the number of
PIC microcontroller I/O pins used. Two on-board variable
resistors allow users to experiment with different designs
on the bench.
A complete solution is achieved by interfacing this board
to the PICkit™ 1 Flash Starter Kit (see DS40051) and the
Signal Analysis PICtail Daughter Board (see DS51476).
MCP6SX2 PGA Thermistor PICtail™ Demo Board
(MCP6SX2DM-PICTLTH)
V out = −
R
bw
× V
in
R
aw
r
bw
=
r
ab
×
Wiper Code
# of Resistors
R
aw =
# of Resistors − Wiper Code
×
R
ab
# of Resistors
PICkit™ 1 Serial Analysis
PC Program
PICkit 1
Flash Starter Kit
PC
USB
Hardware
Software
PICkit 1
Firmware
PICA2Dlab.hex
Firmware
14
14
Signal Analysis
PICtail Daughter Board
MCP6SX2 PGA Thermistor
PICtail Demo Board
PGA
MCP6S22
Thermistor
Te
mperature
MCP6SX2 PGA Thermistor
PICtail™ Demo Board
+5
Test Point
GND
Test Point
CH0 Input
Test Point
CH1 Input
Test Point
Thermistor
Voltage
Divider
Signal Analysis
PICtail Daughter Board
PICkit™ 1
Flash Starter Kit
Serial
EEPROM
PIC16F684
ADC
PIC16F745
GND
+5
4
SPI Bus
V
OUT
4
SPI™ Bus
GND
+5
to PC
USB
Programmable Gain
Programmable Amplifier Gain Using a
Digital Potentiometer
Many sensors require their signal to be amplified before
being converted to a digital representation. This signal
gain may be done with and operational amplifier. Since
all sensors will have some variation in their operational
characteristics, it may be desirable to calibrate the gain
of the operational amplifier to ensure an optimal output
voltage range.
The figure below shows two inverting amplifier with
programmable gain circuits. The generic circuit (a) where
R
1
, R
2
, and Pot
1
can be used to tune the gain of the
inverting amplifier, and the simplified circuit (b) which
removes resistors R
1
and R
2
and just uses the digital
potentiometers Raw and Rbw ratio to control the gain.
The simplified circuit reduces the cost and board area
but there are trade-offs (for the same resistance and
resolution), Using the R
1
and R
2
resistors allows the
range of the gain to be limited and therefore each digital
potentiometer step is a fine adjust within that range.
While in the simplified circuit, the range is not limited and
therefore each digital potentiometer step causes a larger
variation in the gain.
The following equation shows how to calculate the gain
for the simplified circuit (figure below). The gain is the
ratio of the digital potentiometers wiper position on the
R
ab resistor ladder. As the wiper moves away from the
midscale value, the gain will either become greater then
one (as wiper moves towards Terminal A), or less then one
(as wiper moves towards Terminal B).
Inverting Amplifier with Programmable Gain Circuits
Generic Circuit (a)
Pot1
V
OUT
VIN
R2R1
W
AB
Simplified Circuit (b)
Pot1
VOUT
Input
CF
W
AB
Note 1: A general purpose op amp, such as the MCP6001.
Op Amp
(1)
+
–
CF
Op Amp
(1)
+
–