3–284
Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Prepared by: Warren Schultz
Discrete Applications Engineering
INTRODUCTION
Amplifiers for interfacing Semiconductor Pressure Sensors
to electronic systems have historically been based upon
classic instrumentation amplifier designs. Instrumentation
amplifiers have been widely used because they are well
understood standard building blocks that also work
reasonably well. For the specific job of interfacing
Semiconductor Pressure Sensors to today’s mostly digital
systems, other circuits can do a better job. This application
note presents an evolution of amplifier design that begins with
a classic instrumentation amplifier and ends with a simpler
circuit that is better suited to sensor interface.
INTERFACE AMPLIFIER REQUIREMENTS
Design
determined by the sensor’s output characteristics, and the
zero to 5 V input range that is acceptable to microcomputer
A/D converters. Since the sensor’s full scale output is typically
tens of millivolts, the most obvious requirement is gain. Gains
from 100 to 250 are generally needed, depending upon bias
voltage applied to the sensor and maximum pressure to be
measured. A differential to single–ended conversion is also
requirements
for
interface
amplifiers
are
required in order to translate the sensor’s differential output
into a single ended analog signal. In addition, level shifting is
necessary to convert the sensor’s 1/2 B+ common mode
voltage to an appropriate DC level. For microcomputer A/D
inputs, generally that level is from 0.3 – 1.0 V. Typical design
targets are 0.5 V at zero pressure and enough gain to produce
4.5 V at full scale. The 0.5 V zero pressure offset allows for
output saturation voltage in op amps operated with a single
supply (VEE = 0). At the other end, 4.5 V full scale keeps the
output within an A/D converter’s 5 V range with a comfortable
margin for component tolerances. The resulting 0.5 to 4.5 V
single–ended analog signal is also quite suitable for a variety
of other applications such as bar graph pressure gauges and
process monitors.
CLASSIC INSTRUMENTATION AMPLIFIER
A classic instrumentation amplifier is shown in Figure 1.
This circuit provides the gain, level shifting and differential to
single–ended conversion that are required for sensor
interface. It does not, however, provide for single supply
operation with a zero pressure offset voltage in the desired
range.
–
+
–
+
+
–
8
10
9
R2
1 k
3
2
11
1
5
6
47
* NOTE: FOR MPX2020 R10 = 150 OHMS
R 3 1 k
R5
1 k
R9 15 k
R8 15 k
+
–
U1A
MC33274
U1B
MC33274
VEE
R10
240*
VCC
OUTPUT
R4 1k
U1C
MC33274
C3 0.001
μ
F
Figure 1. Classic Instrumentation Amplifier
SEMICONDUCTOR APPLICATION NOTE
REV 2
F
Freescale Semiconductor, Inc.
n
.