AN990

AN990
Analog Sensor Conditioning Circuits – An Overview
Author:
Kumen Blake
Microchip Technology Inc.
INTRODUCTION
Target Audience
This application note is intended for hardware design
engineers that need to condition the output of common
analog sensors.
Goals
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Review sensor applications (e.g., temperature)
Review sensor types (e.g., voltage output)
Show various conditioning circuits
Give technical references
Description
Analog sensors produce a change in an electrical
property to indicate a change in its environment. This
change in electrical property needs to be conditioned
by an analog circuit before conversion to digital.
Further processing occurs in the digital domain but is
not addressed in this application note.
The applications mentioned are:
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Electrical
Magnetic
Temperature
Humidity
Force, Weight, Torque and Pressure
Motion and Vibration
Flow
Fluid Level and Volume
Light and Infrared (IR)
Chemistry
For each type of electrical property, commonly used
conditioning circuits are shown. Each circuit has an
accompanying list of advantages and disadvantages,
and a list of sensor types appropriate for that circuit.
The electrical properties covered are:
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In addition, circuit and firmware concerns common to
many embedded designs are briefly mentioned:
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Input Protection
Sensor Failure Detection
Filtering
Analog-to-Digital (A-to-D) Conversion
Correction of Results
References to documents that treat these subjects in
more depth have been included in the “References”
section.
SENSOR APPLICATIONS
This section reviews a few analog sensor applications.
For each application, a list of common sensor types is
given for convenience. A good resource for many of
these applications is OMEGA® Engineering’s
handbooks [1, 2].
There are many more analog sensors than the ones
discussed in this application note. For example:
• Time/frequency counters [14]
• Distance ranging sensor [25]
• Current sensing transformer [6]
Emphasis is placed on the electrical behavior of the
various sensors. It is necessary to know this
information when selecting an appropriate sensor
conditioning circuit.
Electrical
These applications measure the state at some point in
an electrical circuit. They include monitoring the
condition of a crucial electrical circuit or power source.
TABLE 1:
ELECTRICAL APPLICATIONS
Sensor
Electrical Parameter
Voltage
Voltage
Current
Current
Charge
Charge
Voltage
Current
Resistance
Capacitance
Charge
© 2005 Microchip Technology Inc.
DS00990A-page 1
AN990
Magnetic
Motion and Vibration
These sensors are used to detect magnetic field
strength and/or direction. They are commonly used in
compasses and motor control [6].
Some common analog motion and vibration sensors
are listed in Table 6. In many cases, more integrated
solutions are available.
TABLE 2:
TABLE 6:
MAGNETIC APPLICATIONS
Sensor
Electrical Parameter
Hall effect [6]
Voltage
Magneto-resistive
Resistance
MOTION AND VIBRATION
APPLICATIONS
Sensor
Electrical Parameter
LVDT [10]
AC Voltage
Piezo-electric
Voltage or Charge
Temperature
Microphone
Voltage
The most common sensor application is temperature
measurement. Some common sensors are listed in
Table 3. Overviews of temperature sensors can be
found in the references [14, 15].
Motor Sensors [6]
Voltage, Resistance,
Current, ...
Ultrasonic Distance [25]
Time
IC Accelerometers
Voltage
TABLE 3:
TEMPERATURE
APPLICATIONS
Sensor
Electrical Parameter
Flow
Many different approaches are used for measuring the
flow of liquids and gases. A short sample is shown in
Table 7.
Thermocouple [19, 20]
Voltage
RTD [18]
Resistance
Thermistor [16, 17]
Resistance
IC
Voltage
IR Thermal Sensor
Current
Magnetic Flow Meter
AC Voltage
Thermo Piles
Voltage
Mass Flow Meter
(temperature)
Resistance
Humidity
Ultrasound/Doppler
Frequency
Two common ways to measure humidity are listed in
Table 4. It is often necessary to compensate for
temperature in these applications.
Hot-wire Anemometer
[24]
Resistance
Mechanical Transducer
(e.g., turbine)
Voltage, ...
TABLE 4:
TABLE 7:
Sensor
Electrical Parameter
HUMIDITY APPLICATIONS
Sensor
Electrical Parameter
Capacitive
Capacitance
Infrared (IR)
Current
Force, Weight, Torque, and Pressure
Fluid Level and Volume
Table 8 gives several examples of fluid level sensors.
Fluid volume in a rigid container can be calculated from
the level.
TABLE 8:
The sensors in this section measure a mechanical
force or strain. Common types are listed in Table 5.
TABLE 5:
FLOW APPLICATIONS
FORCE, WEIGHT, TORQUE,
AND PRESSURE
APPLICATIONS
Sensor
Electrical Parameter
Strain Gage [8 - 10]
Resistance
Load Cell
Resistance
Piezo-electric
Voltage or Charge
Mechanical Transducer
Resistance, Voltage, ...
DS00990A-page 2
FLUID LEVEL AND VOLUME
APPLICATIONS
Sensor
Electrical Parameter
Ultrasound
Time
Mechanical Transducer
Resistance, Voltage, ...
Capacitive
Capacitance
Switch (e.g., vibrating)
On/Off
Thermal
—
© 2005 Microchip Technology Inc.
AN990
Light and Infrared (IR)
Advantages
Light and IR are used to detect the presence of objects
(e.g., people in a burglar alarm) and reduction in
visibility (smoke and turbidity detectors).
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•
•
•
TABLE 9:
LIGHT AND IR
APPLICATIONS
Sensor
Disadvantages
Electrical Parameter
Photodiode [22, 23]
High input impedance
Low bias current (CMOS op amps)
Positive gain
Simplicity
Current
• Limited input voltage range
• Input stage distortion
• Amplifies common mode noise
Chemistry
Sensor Examples
Table 10 gives a short list of sensors that detect
chemical conditions.
• Thermocouple
• Thermo pile
• Piezo-electric film
TABLE 10:
CHEMISTRY APPLICATIONS
Sensor
Electrical Parameter
pH Electrode
Voltage (with high output
impedance)
Solution Conductivity
Resistance
CO Sensor
Voltage or Charge
Turbidity (photodiode)
Current
Colorimeter (photodiode)
Current
BUFFER FOR HIGH IMPEDANCE VOLTAGE
SOURCE
This circuit requires a FET input op amp (e.g., CMOS
input); see Figure 2. The FET input gives very high
input impedance and very low input bias current, especially at room temperature (the ESD diodes conduct
more current at higher temperatures). The operational
amplifier (op amp) is used as a non-inverting amplifier.
VDD
BASIC SIGNAL CONDITIONING
CIRCUITS
MCP6XXX
VSEN
This section is organized by the sensor’s electrical
property. For each sensor electrical property listed, one
or more conditioning circuits are shown. Advantages,
disadvantages and sensor examples are listed for each
circuit.
VOUT
R1
R2
FET Input Op Amp
Voltage Sensors
The circuits in this section condition a voltage produced
by a sensor.
FIGURE 2:
Non-inverting Gain Amplifier
for High-Impedance Sensors with Voltage Output.
NON-INVERTING GAIN AMPLIFIER
Advantages
Figure 1 shows a non-inverting gain amplifier using an
op amp. It presents a high impedance to the sensor (at
VSEN) and produces a positive gain from VSEN to VOUT.
•
•
•
•
VDD
Disadvantages
MCP6XXX
VSEN
VOUT
R1
Very high input impedance
Very low bias current (CMOS op amps)
Positive gain
Simplicity
R2
• Limited input voltage range
• Input stage distortion
• Amplifies common mode noise
Sensor Example
• pH electrode
FIGURE 1:
Non-inverting Gain Amplifier.
© 2005 Microchip Technology Inc.
DS00990A-page 3
AN990
The pH electrode’s impedance is a function of temperature and can be quite large. Its output voltage is
proportional to absolute temperature.
INVERTING GAIN AMPLIFIER
Figure 3 shows an inverting gain amplifier using an op
amp. It presents an impedance of R1 to the sensor (at
VSEN) and produces a negative gain from VSEN to
VOUT.
VOUT
VSEN
• Remote thermocouple
• Wheatstone bridge
INSTRUMENTATION AMPLIFIER
R2
Inverting Gain Amplifier.
Advantages
•
•
•
•
Disadvantages
Sensor Examples
MCP6XXX
FIGURE 3:
• Resistive isolation from the source
• Large input voltage range is possible
• Rejects common mode noise; it is good for
remote sensors
• Simplicity
• Resistive loading of the source
• Input stage distortion
VDD
R1
Advantages
Resistive isolation from the source
Large input voltage range is possible
Virtually no input stage distortion
Simplicity
Figure 5 shows an instrumentation amplifier circuit that
conditions a remote voltage sensor. The input resistors
provide isolation and detection of sensor open-circuit
failure. It amplifies the input difference voltage
(VSEN+ – VSEN–) and rejects common mode noise.
VDD
R1
R2
Disadvantages
VSEN+
• Resistive loading of the source
• Inverting gain
• Amplifies common mode noise
VSEN–
R2
R1
VDD
Instrumentation
Amplifier
VOUT
VREF
Sensor Examples
• Thermo pile
• High-side (VDD) voltage sensor
FIGURE 5:
Instrumentation Amplifier.
Advantages
DIFFERENCE AMPLIFIER
Figure 4 shows a difference amplifier using an op amp.
It presents an impedance of R1 to each end of the
sensor (VSEN+ and VSEN–) and amplifies the input
difference voltage (VSEN+ – VSEN–).
• Excellent rejection of common mode noise; it is
great for remote sensors
• Resistive isolation from the source
• Detection of sensor failure
Disadvantages
R1
R2
• Resistive loading of the source
• Cost
VSEN+
VDD
Sensor Examples
MCP6XXX
VOUT
VSEN–
R1
FIGURE 4:
DS00990A-page 4
R2
• Remote thermocouple
• Remote RTD (with a current source or voltage
divider to produce a voltage from the RTD)
• Wheatstone bridge
- Strain gage
- Pressure sensor
Difference Amp.
© 2005 Microchip Technology Inc.
AN990
VARIABLE GAIN FOR WIDE DYNAMIC RANGE
AND NON-LINEAR SENSORS
Figure 6 shows a Programmable Gain Amplifier (PGA)
used to condition multiple sensors. These PGAs (e.g.,
MCP6S22) allow the user to select an input sensor and
gain with the SPI™ bus. It can also help linearize
non-linear sensors (e.g., a thermistor; see [16]).
VDD
MCP6SX2
VSEN
VDD
CH0
CH1
VREF
To other
sensor
FIGURE 6:
Amplifer.
The circuits in this section condition a current produced
by a sensor.
RESISTIVE DETECTOR
Figure 7 shows a resistor (R1) that converts the sensor
current (ISEN) to a voltage (see [6]), as well as a
difference amplifier that amplifies the voltage across
the resistor while rejecting common mode noise.
.
ISEN
VOUT
SPI™ Control
Current Sensors
R2
R3
VOUT
4
VSS
VDD
MCP6XXX
R1
VOUT
Programmable Gain
Advantages
• Multiple sensors (input MUX)
• CMOS input (high impedance and low bias
current)
• Digital control (SPI) of input and gain
• Linearization of non-linear sources
Disadvantages
• Input stage distortion
• Amplifies common mode noise
• Needs microcontroller unit (MCU) and firmware
Sensor Examples
• Thermistor (with voltage divider to convert
resistance to voltage)
• Thermo pile
• Piezo-electric film
© 2005 Microchip Technology Inc.
R2
R3
R1 << R2
FIGURE 7:
Current Sensor.
Advantages
• Good rejection of common mode noise
• Resistive isolation from the source
• Wide input voltage range
Disadvantages
• Resistive loading of the source
• Input stage distortion
Sensor Examples
• High-side (VDD) current sensor
• AC mains (line) current
DS00990A-page 5
AN990
TRANSIMPEDANCE AMPLIFIER
LOGARITHMIC AMPLIFIER (LOG AMP)
Figure 8 shows a transimpedance amplifier (R1 and the
op amp) that converts the sensor current (ISEN) to a
voltage. The capacitor C1 is sometimes needed to
stabilize the amplifier when the source has a large
capacitance (e.g., see [5]).
Figure 8 shows a logarithmic amplifier (D1A and the op
amp) that converts the sensor current (ISEN) to a
voltage proportional to the logarithm of the current. R1
maintains negative feedback when ISEN is small or
negative. D1B is used to correct D1A for temperature
changes.
ISEN
R1
VOUT
VDD
R2
ISEN
D1A
VOUT
C1
R1
VDD
VDD
R2
MCP6XXX
VDD
R2
VDD
MCP6XXX
FIGURE 8:
Transimpedance Amplifier.
R2
R3
VCOR
Advantages
• Good impedance buffering of source
• Simplicity
Disadvantages
D1B
D1A and D1B are a matched pair
in the same package.
• Design may need to be stabilized
FIGURE 9:
Sensor Examples
When the source (ISEN) has both polarities, add a diode
in parallel with R1 and D1A, and with the opposite
polarity to D1A.
• IR smoke detector
• Photodiode
• Photodetector
Logarithmic Amplifier.
Advantages
• Wide dynamic range of currents
• Good impedance buffering of source
• Simplicity
Disadvantages
• Needs temperature correction
Sensor Example
• Photodiode (e.g., PWM encoded digital signal)
DS00990A-page 6
© 2005 Microchip Technology Inc.
AN990
Resistive Sensors
Voltage Divider and Variable Gain
The sensors in this section produce a change in resistance. There are four basic strategies shown here for
converting this resistance into a measurable electrical
quantity:
Figure 11 shows a voltage divider (RSEN and R1) that
converts the sensor resistance to a voltage. The PGA
buffers the voltage divider for further signal processing
and can be set to different gains when the sensor is
non-linear.
•
•
•
•
Resistance-to-voltage conversion
Resistance-to-current conversion
RC decay
Oscillator frequency
VDD
VDD
RESISTANCE-TO-VOLTAGE CONVERSION
The first strategy for conditioning a resistive sensor is
to produce a voltage that is a function of the change in
resistance.
Voltage Divider
Figure 10 shows a voltage divider (RSEN and R1) that
converts the sensor resistance to a voltage. The op
amp buffers the voltage divider for further signal
processing. This approach has been used in AN867
and AN897 [21, 16].
CH0
CH1
RSEN
SPI™ Control
VREF
VOUT
4
VSS
To other
sensor
FIGURE 11:
Voltage Divider with PGA.
Advantages
• Linearization of non-linear sensors
• Ratiometric output (with an ADC using VDD as its
reference voltage)
• Multiplexing several sensors
• Detection of open sensor (failure)
VDD
RSEN
MCP6SXX
VDD
VOUT
R1
VDD
MCP6XXX
VOUT
R1
FIGURE 10:
Op Amp.
Voltage Divider with
Disadvantages
• Poor common mode noise rejection
• Needs a controller and firmware
• Voltage is a non-linear function of resistance
Sensor Example
Advantages
• Thermistor
• Simplicity
• Ratiometric output (with an Analog-to-Digital
Converter (ADC) using VDD as its reference
voltage)
• Detection of open sensor (failure)
Wheatstone Bridge
Figure 12 shows a Wheatstone bridge that converts a
change in resistance to a change in differential voltage.
The op amp amplifies the difference voltage.
VDD
Disadvantages
• Poor common mode noise rejection
• Voltage is a non-linear function of resistance
R1
RSEN
VDD
MCP6XXX
Sensor Examples
• Thermistor
• RTD
• Magneto-resistive compass
VOUT
RSEN
R1
R2
FIGURE 12:
Op Amp Circuit.
© 2005 Microchip Technology Inc.
Wheatstone Bridge – Single
DS00990A-page 7
AN990
Advantages
Floating Current Source
• Good rejection of common mode noise
• Ratiometric output (with an ADC using VDD as its
reference voltage)
• Simplicity
• Detection of open sensor (failure)
Figure 14 shows a circuit that provides a current source
(ISEN) that accurately converts resistance to voltage.
R1A, R1B, R1, R2, R3 and the op amp form a current
source (Howland current pump). C1 stabilizes this
current source and reduces noise. R4 provides isolation
from ground for remote sensors. The voltage across
RSEN is amplified by a difference amplifier (Figure 4)
which also rejects common mode noise. The voltage on
top of R4 can be used to detect an open (failed) sensor.
Another current source is shown in [3, 18].
Disadvantages
• Gain is a function of RSEN
• Needs a controller and firmware to correct
• Voltage is a non-linear function of resistance
.
VDD
Sensor Examples
• Strain gage
• Pressure sensor
• Magneto-resistive compass
R1A
R2
R1B
VDD
MCP6XXX
Figure 13 shows another Wheatstone bridge circuit.
The instrumentation amplifier amplifies the bridge’s
difference voltage and gives excellent rejection of
common mode noise.
(
R1
VDD
R1
RSEN
R2
ISEN
Instrumentation
Amp
RSEN
VOUT
RSEN
R3
C1
R1
VREF
FIGURE 13:
Wheatstone Bridge –
Instrumentation Amplifier Circuit.
Advantages
• Excellent common mode noise rejection
• Ratiometric output (with an ADC using VDD as its
reference voltage)
• Detection of open sensor (failure)
Disadvantages
• Cost
• Voltage is a non-linear function of resistance
Sensor Examples
• Strain gage
• Pressure sensor
• Magneto-resistive compass
R1 = R1A || R1B
R3 << R2 and RSEN
Diff.
Amp.
VOUT
R4
FIGURE 14:
Howland Current Pump and
Resistive Sensor with Difference Amplifier.
Advantages
• Linearity of resistance to voltage conversion
• Ratiometric output (with an ADC using VDD as its
reference voltage)
Disadvantages
• Cost
• Requires accurate resistors
Sensor Examples
• Thermistor
• RTD
• Hot-wire anemometer
Other implementations are shown in application notes
AN251, AN717 and AN695 [8, 9, 10].
DS00990A-page 8
© 2005 Microchip Technology Inc.
AN990
RESISTANCE-TO-CURRENT CONVERSION
RC DECAY
The second strategy for conditioning a resistive sensor
is to produce a current that is a function of the
resistance. Figure 15 shows the basic strategy, where
the “I-to-V Amplifier” can be a transimpedance amp
(Figure 8) or a logarithmic amp (Figure 9).
The third strategy for conditioning a resistive sensor is
to produce a voltage with a RC decay (single pole
response to a step). The time it takes for the voltage to
decay to a threshold is a measure of the resistance.
VDD
RSEN
VDD
ISEN
R2
I-to-V
Amplifier
VOUT
R2
Figure 16 show a circuit using a MCU circuit that sets a
ratiometric threshold (proportional to VDD). The time is
measured for both R1 and RSEN separately in order to
correct for VDD, C1, and temperature errors. The
PICmicro® MCU provides the switching and control
needed. Application notes AN863, AN512 and AN929
[7, 11, 14] detail variations of this circuit.
PICmicro® MCU
RSEN
P2
FIGURE 15:
Resistance-to-Current
Conversion Circuit.
R1
P1
Advantages
P0
• Ratiometric output (with an ADC using VDD as its
reference voltage)
• Simplicity
C1
FIGURE 16:
RC Decay.
Disadvantages
• Inverting gain
Sensor Example
• Thermistor
Advantages
• Ratiometric correction of VDD, C1 and
temperature errors
• Accurate
• Simple timing measurement
Disadvantages
• PICmicro MCU timing resolution
• Digital noise
• Threshold must be ratiometric
Sensor Example
• Thermistor
© 2005 Microchip Technology Inc.
DS00990A-page 9
AN990
OSCILLATOR FREQUENCY
Capacitive Sensors
The fourth strategy for conditioning a resistive sensor is
to measure a change in oscillation frequency;
Figure 17 shows one implementation. It is a state
variable oscillator using resistors, capacitors, op amps
and a comparator. Its operation and design are detailed
in application notes AN866 and AN895 [4, 12].
The sensors in this section produce a change in
capacitance. There are four basic strategies shown
here for converting this capacitance into a measurable
electrical quantity:
C4
R2
VDD
C2
VDD
R7
R3
MCP6XXX
C1
MCP6XXX
MCP6XXX
R1
The first strategy for conditioning a capacitive sensor is
to produce a voltage with a RC decay (single pole
response to a step). The time it takes for the voltage to
decay to a threshold is a measure of the capacitance.
Figure 18 measures this time, where the threshold is
proportional to VDD. R1 has a low temperature
coefficient to minimize temperature errors. The
PICmicro® MCU provides the switching and control
needed. AN863, AN512 and AN929 [7, 11, 14] detail a
similar circuit.
VDD
R8
VOUT
R5
VDD
VDD
RC decay
Oscillator frequency
Integration of current
Wheatstone bridge
RC DECAY
R4
VDD
•
•
•
•
PICmicro® MCU
R1
P1
R6
C5
VDD/2
P0
CSEN
MCP65XX
MCP6XXX
FIGURE 17:
State Variable Oscillator.
FIGURE 18:
RC Decay.
Advantages
Advantages
• Accuracy (with calibration)
• Good startup
• Easy processing using a PICmicro® MCU
• Ratiometric correction of VDD and temperature
errors
• Accurate
• Simple timing measurement
Disadvantages
• Cost
• Design complexity
Sensor Examples
• RTD
• Hot-wire anemometer
Disadvantages
• PICmicro MCU timing resolution
• Digital noise
• Threshold must be ratiometric
Sensor Examples
• Capacitive humidity sensor
• Capacitive touch sensor
• Capacitive tank level sensor
DS00990A-page 10
© 2005 Microchip Technology Inc.
AN990
OSCILLATOR FREQUENCY
SINGLE SLOPE INTEGRATING DETECTOR
The second strategy for conditioning a capacitive
sewnsor is to measure a change in oscillation
frequency. The multi-vibrator (oscillator) in Figure 19
produces a change in oscillation frequency as a
function of capacitance. Its operation and design is
detailed in AN866 and AN895 [4, 12].
The third strategy for conditioning a capacitive sensor
is to integrate a current and measure the elapsed time
to reach a voltage threshold. Figure 20 shows a
single-slope integrating detector. Switch SW1,
controlled by the PICmicro® MCU, zeros the voltage
across CSEN at the start of the integration period. The
voltage at the output of the op amp linearly increases
with time; the rate of increase is set by VREF and R1.
The comparator at the output, which can be on the
PICmicro MCU, trips at a time proportional to CSEN.
AN611 [13] discusses a similar circuit.
CSEN
R1
VDD
MCP65XX
VDD
VOUT
R2
FIGURE 19:
VREF
to MCU
R4
R1
Multi-vibrator (oscillator).
SW1
Advantages
• Cost
• Ratiometric operation
• Easy processing using a PICmicro® MCU
CSEN
MCP65XX
R3
VDD
MCP65XX
VDD
FIGURE 20:
Detector.
Single-slope Integrating
Advantages
Disadvantages
• Reduced accuracy
• Easy processing using a PICmicro® MCU
• Accuracy depends on VREF and R1
Sensor Examples
Disadvantages
• Capacitive humidity sensor
• Capacitive touch sensor
• Capacitive tank level sensor
• Cost
Sensor Examples
• Capacitive humidity sensor
• Capacitive touch sensor
• Capacitive tank level sensor
© 2005 Microchip Technology Inc.
DS00990A-page 11
AN990
CAPACITIVE WHEATSTONE BRIDGE
Charge Sensors
The fourth strategy for conditioning a capacitive sensor
is to convert its impedance, at a specific frequency, to
a voltage using a Wheatstone bridge. Figure 21
produces a change in differential voltage as a function
of change in capacitance. An AC voltage source must
drive the bridge; its frequency needs to be stable and
accurate. R1 can be a digital potentiometer (digi-pot)
that is controlled to zero-out the differential voltage, or
it can be a regular resistor. R3 provides a means to bias
the instrumentation amp correctly, and to keep the
node between the capacitors from drifting over time. It
needs to be much larger than C2’s impedance (1/jωC2);
the divider equation can be corrected for this
resistance, if necessary.
Figure 22 shows a simplified model of a “charge
sensor.” It is a capacitive source that produces AC
energy as a function of a change in the environment.
VAC
CSEN
R1 Instrumentation
Amplifier
VOUT
C2
R2
R3
VREF
CSEN
VSEN
FIGURE 22:
Model.
Simplified Charge Sensor
Figure 23 shows a charge amplifier (C1 and the op
amp) that converts the sensor energy (charge) to an
output voltage. R1 provides a bias path for the inverting
input of the op amp, and creates a high-pass filter pole
(keeps the inverting input of the op amp from drifting
over time). The change in charge of PSEN appears
almost exclusively across C1, which makes this an
accurate way to measure the charge produced by the
sensor.
R1
C1
FIGURE 21:
Bridge.
Capacitive Wheatstone
VOUT
PSEN
VDD
Advantages
R2
• Excellent common mode noise rejection
• Ratiometric output (with an ADC using VDD as its
reference voltage)
• Detection of open or shorted sensor (failure)
FIGURE 23:
• Needs AC stimulus
• Power dissipation
Advantages
• Remote capacitive sensors
- Humidity sensor
- Touch sensor
- Tank level sensor
MCP6XXX
R3
Disadvantages
Sensor Examples
VDD
Charge Amplifier.
• Excellent common mode noise rejection
• Ratiometric output (with an ADC using VDD as its
reference voltage)
• Detection of open or shorted sensor (failure)
Disadvantages
• Needs AC stimulus
• Power dissipation
Sensor Example
• Piezo-electric film
DS00990A-page 12
© 2005 Microchip Technology Inc.
AN990
ADDITIONAL SIGNAL CONDITIONING
A-to-D Conversion
Circuit and firmware concerns common to many
embedded designs are mentioned here.
Many times, the conditioned sensor output is converted
to digital format by an ADC. Many of the circuits in this
application note are ratiometric so that variations in
power supply are corrected at the ADC (e.g., Wheatstone bridges). Others circuits use an absolute
reference for the ADC.
Input Protection
Sensor inputs need to be protected against Electrostatic Discharge (ESD), overvoltage and overcurrent
events; especially if they are remote from the
conditioning circuit. AN929 [14] covers these issues.
Sensor Failure Detection
Some of the circuits in this application note provide
means to detect sensor failure. Other examples are
given in AN929 [14].
Filtering
All of the circuits in this application note also need
output filters [3]. Analog filters are used to improve
ADC performance. When properly designed, they
prevent interference from aliasing (even to DC) and
can reduce the sample frequency requirements (saving
power and MCU overhead). A simple RC filter is good
enough for many applications. More difficult analog filters need to be implemented with active RC filters.
Microchip Technology Inc.’s FilterLab® software [26] is
an innovative tool that simplifies analog active-filter
(using op amps) design. It is available at no cost from
our web site (www.microchip.com). The FilterLab
active-filter software design tool provides full schematic
diagrams of the filter circuit with component values. It
also outputs the filter circuit in SPICE format.
Additional filtering can be performed digitally, if
necessary. A simple averaging of results is usually
good enough.
© 2005 Microchip Technology Inc.
Correction of Results
Sensor errors can be corrected by calibrating each
system. This can be accomplished in hardware (e.g.,
Digi-Pot) or firmware (e.g., calibration constants in
non-volatile memory).
Correction for other environmental parameters may
also be needed. For example, a capacitive humidity
sensor may need correction for temperature. This is
usually easiest to handle in firmware, but can also be
done in hardware.
Non-linear sensors need additional correction. They
may use polynomials or other mathematical functions
in the MCU, to produce a best estimate of the parameter of interest. It is also possible to use a linear interpolation table in firmware; AN942 [27] gives one
implementation.
SUMMARY
This application note is intended to assist circuit
designers select a circuit topology for common sensor
types. Common sensor applications are listed and
described. Many basic signal-conditioning circuits are
shown. Sensor-conditioning circuitry, and firmware
common to many embedded designs, are briefly
mentioned. The “References” section points to other
resources that cover particular topics in detail.
DS00990A-page 13
AN990
REFERENCES
General References
[1] “The OMEGA® Made in the USA Handbook™,”
Vol. 1, OMEGA Engineering, Inc., 2002.
[2] “The OMEGA® Made in the USA Handbook™,”
Vol. 2, OMEGA Engineering, Inc., 2002.
[3] AN682, “Using Single Supply Operational
Amplifiers in Embedded Systems,” Bonnie Baker;
Microchip Technology Inc., DS00682, 2000.
[4] AN866, “Designing Operational Amplifier Oscillator
Circuits For Sensor Applications,” Jim Lepkowski;
Microchip Technology Inc., DS00866, 2003.
Current Sensors
[5] AN951, “Amplifying High-Impedance Sensors –
Photodiode Example,” Kumen Blake and Steven Bible;
Microchip Technology Inc., DS00951, 2004.
[6] AN894, “Motor Control Sensor Feedback Circuits,”
Jim Lepkowski; Microchip Technology Inc., DS00894,
2003.
Resistor Sensors
[7] AN863, “A Comparator Based Slope ADC,” Joseph
Julicher; Microchip Technology Inc., DS00863, 2003.
[8] AN251, “Bridge Sensing with the MCP6S2X
PGAs,” Bonnie C. Baker; Microchip Technology Inc.,
DS00251, 2003.
[9] AN717, “Building a 10-bit Bridge Sensing Circuit
using the PIC16C6XX and MCP601 Operational
Amplifier,” Bonnie C. Baker; Microchip Technology Inc.,
DS00717, 1999.
[10] AN695, “Interfacing Pressure Sensors to
Microchip’s Analog Peripherals,” Bonnie Baker;
Microchip Technology Inc., DS00695, 2000.
[11] AN512, “Implementing Ohmmeter/Temperature
Sensor,” Doug Cox; Microchip Technology Inc.,
DS00512, 1997.
[12] AN895 “Oscillator Circuits For RTD Temperature
Sensors,” Ezana Haile and Jim Lepkowski; Microchip
Technology Inc., DS00895, 2004.
[15] AN679, “Temperature Sensing Technologies,”
Bonnie C. Baker; Microchip Technology Inc., DS00679,
1998.
[16] AN897; “Thermistor Temperature Sensing with
MCP6SX2 PGAs,” Kumen Blake and Steven Bible;
Microchip Technology Inc., DS00897, 2004.
[17] AN685,
“Thermistors
in
Single
Supply
Temperature Sensing Circuits,” Bonnie C. Baker;
Microchip Technology Inc., DS00685, 1999.
[18] AN687, “Precision Temperature-Sensing With
RTD Circuits,” Bonnie C. Baker; Microchip Technology
Inc., DS00687, 2003.
[19] AN684, “Single Supply Temperature Sensing with
Thermocouples,” Bonnie C. Baker; Microchip
Technology Inc., DS00684, 1998.
[20] AN844, “Simplified Thermocouple Interfaces and
PICmicro® MCUs,” Joseph Julicher; Microchip
Technology Inc., DS00844, 2002.
[21] AN867,
“Temperature
Sensing
With
A
Programmable Gain Amplifier,” Bonnie C. Baker;
Microchip Technology Inc., DS00867, 2003.
Other Sensors
[22] AN865, “Sensing Light with a Programmable Gain
Amplifier,” Bonnie C. Baker; Microchip Technology Inc.,
DS00865, 2003.
[23] AN692, “Using a Digital Potentiometer to Optimize
a Precision Single-Supply Photo Detection Circuit,”
Bonnie C. Baker; Microchip Technology Inc., DS00692,
2004.
[24] TB044, “Sensing Air Flow with the PIC16C781,”
Ward Brown; Microchip Technology Inc., DS91044,
2002.
[25] AN597, “Implementing Ultrasonic Ranging,”
Robert Schreiber; Microchip Technology Inc.,
DS00597, 1997.
Signal Conditioning
[26] FilterLab® 2.0 User’s Guide;”
Technology Inc., DS51419, 2003.
Microchip
[27] AN942, “Piecewise Linear Interpolation on
PIC12/14/16 Series Microcontrollers,” John Day and
Steven Bible; Microchip Technology Inc., 2004.
Capacitance Sensors
[13] AN611, “Resistance and Capacitance Meter
Using a PIC16C622,” Rodger Richie; Microchip
Technology Inc., DS00611, 1997.
Temperature Sensors
[14] AN929, “Temperature Measurement Circuits for
Embedded Applications,” Jim Lepkowski; Microchip
Technology Inc., DS00929, 2004.
DS00990A-page 14
© 2005 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
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•
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•
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Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
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© 2005 Microchip Technology Inc.
DS00990A-page 15
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DS00990A-page 16
© 2005 Microchip Technology Inc.
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