Datasheets AN990 Datasheet

AN990

Analog Sensor Conditi oning Circuits – An Ov erview

Author: Kumen Blake

Microchip Technology Inc.

INTRODUCTION

Target Audience

This application note is intended for hardware design
engineers that need to co nditio n the output of commo n
analog sensors.

Goals

• 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:

• 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:

•Voltage

• Current

• Resistance

• Capacitance

•Charge

In addition, circuit and firmware concerns common to
many embedded designs are briefly mentioned:

• 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
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 circu it.

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

®

Engineering’s

© 2005 Microchip Technology Inc. DS00990A-page 1

AN990

Magnetic

These sensors are used to detect magnetic field
strength and/or direction. They are commonly used in
compasses and motor control [6].

TABLE 2: MAGNETIC APPLICATIONS

Sensor Electrical Parameter

Hall effect [6] Voltage
Magneto-resistive Resistance

Temperature

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].

TABLE 3: TEMPERATURE

APPLICATIONS

Sensor Electrical Parameter

Thermocouple [19, 20] Voltage
RTD [18] Resistance
Thermistor [16, 17] Resistance
IC Voltage
IR Thermal Sensor Current
Thermo Piles Voltage

Humidity

Two common ways to measure humidity are listed in
Table 4. It is often necessary to compensate for
temperature in these applic ati on s.

TABLE 4: HUMIDITY APPLICATIONS

Sensor Electrical Parameter

Capacitive Capacitance
Infrared (IR) Current

Motion and Vibration

Some common analog motion and vibration sensors
are listed in Table 6. In many cases, more integrated
solutions are available.

TABLE 6: MOTION AND VIBRATION

APPLICATIONS

Sensor Electrical Parameter

LVDT [10] AC Voltage
Piezo-electric Voltage or Charge
Microphone Voltage
Motor Sensors [6] Voltage, Resistance,

Current, ...
Ultrasonic Distance [25] Time
IC Accelerometers Voltage

Flow

Many different approaches are used for measuring the
flow of liquids and gases. A short sample is shown in
Table 7.

TABLE 7: FLOW APPLICATIONS

Sensor Electrical Parameter

Magnetic Flow Meter AC Voltage
Mass Flow Meter

(temperature)
Ultrasound/Doppler Frequency
Hot-wire Anemometer

[24]
Mechanical Transducer

(e.g., turbine)

Resistance

Resistance

Volt a ge, ...

Fluid Level and Volume

Table 8 gives several examples of fluid level sensors.
Fluid volume in a rigid cont ainer can be calcu lated from
the level.

Force, Weight, Torque, and Pressure

The sensors in this section measure a mechanical
force or strain. Common types are listed in Table 5.

TABLE 5: FORCE, WEIGHT, TORQUE,

AND PRESSURE
APPLICATIONS

Sensor Electrical Parameter

Strain Gage [8 - 10] Resistance
Load Cell Resistance
Piezo-electric Volt a ge or Cha rge
Mechanical Transducer Resistance, Voltage, ...

DS00990A-page 2 © 2005 Microchip Technology Inc.

TABLE 8: FLUID LEVEL AND VOLUME

APPLICATIONS

Sensor Electrical Parameter

Ultrasound Time
Mechanical Transducer Resistance, Voltage, ...
Capacitive Capacitance
Switch (e.g., vibrating) On/Off
Thermal —

AN990

Light and Infrared (IR)

Light and IR are used t o detec t the pre sence of obje ct s
(e.g., people in a burglar alarm) and reduction in
visibility (smoke and turb idity detectors).

TABLE 9: LIGHT AND IR

APPLICATIONS

Sensor Electrical Parameter

Photodiode [22, 23] Current

Chemistry

Table 10 gives a short list of sensors that detect
chemical conditions.

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

Advantages

• High input impedance

• Low bias current (CMO S op amps)

• Positive gain

• Simplicity

Disadvantages

• Limited input voltage range

• Input stage distortion

• Amplifies common mo de nois e

Sensor Examples

• Thermocouple

• Thermo pile

• Piezo-ele ctric film

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 ve ry low in put bia s curre nt, esp e­cially at room temperature (the ESD diodes conduct
more current at higher temperatures). The operational
amplifier (op amp) is used as a non-inverting amplifier.

BASIC SIGNAL CONDITIONING
CIRCUITS

This section is organized by the sensor’s electrical
property . For eac h sensor electri cal property liste d, one
or more conditioning circuits are shown. Advantages,
disadvantage s and sen sor examples are listed for each
circuit.

Voltage Sensors

The circuits in this se ction cond ition a vol tage p roduced
by a sensor.

NON-INVERTING GAIN AMPLIFIER

Figure 1 shows a non-inverting gain amplifier using an
op amp. It presents a hi gh impe dance to the sens or (at

) and produces a positive gain from V

V

SEN

V

DD

V

SEN

R

1

MCP6XXX

R

2

FIGURE 1: Non-inverting Gain Amplifier.

SEN

V

OUT

to V

OUT

V

DD

V

SEN

R

1

FET Input Op Amp

MCP6XXX

R

2

V

OUT

FIGURE 2: Non-inverting Gai n Amplifier
for High-Impedance Sensors with Voltage Output.

Advantages

• Very high input impedance

• Very low bias curre nt (CMOS op amps)

.

• Positive gain

• Simplicity

Disadvantages

• Limited input voltage range

• Input stage distortion

• Amplifies common mo de nois e

Sensor Example

• pH electrode

© 2005 Microchip Technology Inc. DS00990A-page 3

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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 R

) and produces a negative gain from V

V

SEN

V

.

OUT

V

DD

to the sensor (at

1

SEN

to

MCP6XXX

V

OUT

V

SEN

R

1

R

2

FIGURE 3: Inverting Gain Amplifier.

Advantages

• Resistive isolation from the source

• Large input voltage range is possible

• Virtually no input stage distortion

• Simplicity

Disadvantages

• Resistive loading of the source

• Inverti ng gain

• Amplifies common mode noise

Advantages

• Resisti ve isolation from the source

• Large input voltage range is possible

• Rejects common mo de noi se ; it is good for
remote sensors

• Simplicity

Disadvantages

• Resistive loading of the source

• Input stage distortion

Sensor Examples

• Remote thermocouple

• Wheatstone bridge

INSTRUMENTATION AMPLIFIER

Figure 5 shows an instrument ation am plifi er circu it that
conditions a remote volt age s enso r. The input resistors
provide isolation and detection of sensor open-circuit
failure. It amplifies the input difference voltage
(V

SEN

V
V

+–V

SEN

SEN

–) and rejects common mode noise.

SEN

V

DD

V

R

1

R

+
–

R

R

1

DD

2

Instrumentation
Amplifier

V

2

REF

V

OUT

Sensor Examples

• Thermo pile

• High-side (VDD) voltage sensor

DIFFERENCE AMPLIFIER

Figure 4 shows a diffe renc e ampli fie r usin g an op am p.
It presents an impedance of R
sensor (V
difference voltage (V

V

SEN

SEN

+

+ and V

SEN

R

1

SEN

+–V

to each end of the

1

–) and amplifies the input

–).

SEN

R

2

V

DD

MCP6XXX

V

OUT

V

–

SEN

R

1

R

2

FIGURE 4: Difference Amp.

FIGURE 5: Instrumentation Amplifier.

Advantages

• Excellent rejection of common mode noise; it is
great for remote sensors

• Resisti ve isolation from the source

• Detection of sensor failure

Disadvantages

• Resistive loading of the source

•Cost

Sensor Examples

• Remote thermocouple

• Remote RTD (with a current source or voltage
divider to produce a voltage from the RTD)

• Wheatstone bridge

- Strain gage

- Pressure sensor

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AN990

V ARIABLE GAIN FOR WIDE DYNAMIC RANGE
AND NON-LINEAR SENSORS

Figure 6 shows a Programmable Gain Amplifie r (PGA)
used to condition multiple sensors. These PGAs (e.g.,
MCP6S22) allow the user to sel ect an input se nsor and
gain with the SPI™ bus. It can also help linearize
non-linear sensors (e.g., a thermistor; see [16]).

V

DD

MCP6SX2

V

V

SEN

CH0
CH1

DD

V

OUT

V

OUT

4

SPI™ Control

V

REF

V

To other

SS

sensor

FIGURE 6: Programmable Gain
Amplifer.

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 voltag e div ider to convert
resistance to voltage)

• Thermo pile

• Piezo-electric film

Current Sensors

The circuits in this sectio n condit ion a cu rrent p roduced
by a sensor.

RESISTIVE DETECTOR

Figure 7 shows a resistor (R1) that converts the se nsor
current (I
difference amplifier that amplifies the voltage across
the resistor while rejecting common mode noise.

.

R1 << R

FIGURE 7: Current Sensor.

Advantages

• Good rejection of common mode noise

• Resisti ve 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

) to a voltage (see [6]), as well as a

SEN

I

SEN

R

2

R

1

R

2

2

R

3

V

DD

MCP6XXX

R

3

V

OUT

© 2005 Microchip Technology Inc. DS00990A-page 5

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TRANSIMPEDANCE AMPLIFIER

Figure 8 shows a transimpedance amplifier (R1 and the
op amp) that converts the sensor current (I
voltage. The capacitor C

is sometimes needed to

1

SEN

) to a

stabilize the amplifier when the source has a large
capacitance (e.g., see [5]).

I

SEN

V

DD

R

2

R

2

R

1

V

OUT

C

1

V

DD

MCP6XXX

FIGURE 8: Transimpedance Amplifier.

Advantages

• Good impedance buffering of source

• Simplicity

Disadvantages

• Design may need to be stabilized

Sensor Examples

• IR smoke detector

• Photodiode

• Photodetector

LOGARITHMIC AMPLIF IER (LOG AMP)

Figure 8 shows a logarithmic ampli fie r (D1A and the op
amp) that converts the sensor current (I

SEN

) to a
voltage proportional to the logarithm of the current. R
maintains negative feedback when I
negative. D

is used to correct D1A for temperature

1B

is small or

SEN

changes.

I

SEN

V

DD

R

2

R

2

D

1A

R

1

V

DD

MCP6XXX

V

OUT

V

DD

R

3

V

COR

D

1B

D1A and D1B are a matched pair
in the same package.

FIGURE 9: Logarithmic Amplifier.

When the source (I
in parallel with R
polarity to D

1A

.

) has both polarities, ad d a diode

SEN

and D1A, and with the opposite

1

Advantages

• Wide dynamic range of currents

• Good impedance buffering of source

• Simplicity

1

Disadvantages

• Needs temperature correction

Sensor Example

• Photodiode (e.g., PWM encoded digital signal)

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AN990

Resistive Sensors

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:

• Resistance-to-voltage conversion

• Resistance-to-current conversion

• RC decay

• Oscillator frequency

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 (R
converts the sensor resistance to a voltage. The op
amp buffers the voltage divider for further signal
processi ng. This approach ha s been used in AN867
and AN897 [21, 16].

V

DD

V

R

SEN

DD

MCP6XXX

R

1

FIGURE 10: Voltage Divider with
Op Amp.

Advantages

• Simplicity

• Ratiometric output (with an Analog-to-Digital
Converter (ADC) using V
voltage)

• Detection of open sensor (failure)

as its reference

DD

and R1) that

SEN

V

OUT

Voltage Divider and Variable Gain

Figure 11 shows a voltage divider (R
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.

V

DD

R

R

1

SEN

CH0
CH1

SPI™ Control

V

REF

V

DD

MCP6SXX

V

DD

V

SS

To other
sensor

V

SEN

OUT

and R1) that

V

OUT

4

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)

Disadvantages

• Poor common mode noise rejection

• Needs a controller and firmware

• Voltage is a non-linear function of resistance

Sensor Example

•Thermistor

Wheatstone Bridge

Figure 12 shows a Wheatstone bridge that converts a
change in resist ance to a chang e in dif ferentia l volt age.
The op amp amplifies the difference voltage.

Disadvantages

• Poor common mode noise rejection

• Voltage is a non-linear function of resistance

V

DD

R

1

R

SEN

V

DD

MCP6XXX

Sensor Examples

•Thermistor

R

SEN

R

1

V

OUT

•RTD

• Magneto-resistive compass
R

2

FIGURE 12: Wheatstone Bridge – Single
Op Amp Circuit.

© 2005 Microchip Technology Inc. DS00990A-page 7

AN990

Advantages

• Good rejection of common mode noise

• Ratiometric output (with an ADC using V

DD

as its

reference voltage)

• Simplicity

• Detection of open sensor (failure)

Disadvantages

• Gain is a function of R

• Needs a controller and firmware to correct

• Voltage is a non-linear function of resistance

SEN

Sensor Examples

• Strain gage

• Pressure sensor

• Magneto-resistive compass
Figure 13 shows another Wheatstone bridge circuit.

The instrumentation amplifier amplifies the bridge’s
difference voltage and gives excellent rejection of
common mode noise.

(

V

DD

R

1

R

Instrumentation

SEN

Amp

Floating Current Source

Figure 14 shows a circuit t hat pr ovid es a cu rrent sourc e

) that accurately converts resistance to voltage.

(I

SEN

, R1B, R1, R2, R3 and the op amp for m a current

R

1A

source (Howland current pump). C
current source and reduces noise. R
from ground for remote sensors. The voltage across
R

is amplified by a difference amplifier (Figure 4)

SEN

which also rejects common mode noise. The voltage on
top of R

can be used to detect an open (failed) sensor.

4

Another current source is shown in [3, 18].

.

V

DD

R

1A

R

1B

R

2

V

DD

MCP6XXX

C

1

R

1

R

R

2

I

SEN

SEN

R

stabilizes this

1

provides isolation

4

3

Diff.

Amp.

V

OUT

V

R

SEN

R

1

V

REF

OUT

FIGURE 13: Wheatstone Bridge –
Instrumentation Amplifier Circuit.

Advantages

• Excellent common mo de noi se reje cti on

• Ratiometric output (with an ADC using V

DD

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
Other implementations are shown in application notes

AN251, AN717 and AN695 [8, 9, 10].

R1 = R1A || R

1B

R3 << R2 and R

SEN

R

4

FIGURE 14: Howland Current Pump and
Resistive Sensor with Difference Amplifier.

Advantages

• Lineari ty 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

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AN990

RESISTANCE-TO-CURRENT CONVERSION

The second strategy for condi tionin g a resis tive se nsor
is to produce a current that is a function of the
resistance. F igure 15 show s the ba sic st rateg y, where
the “I-to-V Amplifier” can be a transimpedance amp
(Figure 8) or a logarithmic amp (Figure 9).

V

DD

R

SEN

R

2

R

2

I

SEN

V

DD

I-to-V

Amplifier

V

OUT

FIGURE 15: Resistance-to-Current
Conversion Circuit.

Advantages

• Ratiometric output (with an ADC using VDD as its
reference voltage)

• Simplicity

Disadvantages

• Inverti ng gain

Sensor Example

•Thermistor

RC DECAY

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.

Figure 16 show a circuit using a MC U circuit that sets a
ratiometric threshold (proportional to V
measured for both R
correct for V
PICmicro

, C1, and temperature errors. The

DD

®

MCU provides the switching and control

and R

1

separately in order to

SEN

). The time is

DD

needed. Application notes AN863, AN512 and AN929
[7, 11, 14] detail variations of this circuit.

PICmicro® MCU

R

SEN

P

2

R1

P

1

P

0

C

1

FIGURE 16: RC Decay.

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

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OSCILLATOR FREQUENCY

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 det ailed
in application notes AN866 and AN895 [4, 12].

C

4

R

V

DD

1

MCP6XXX

R

5

R

6

C

1

V

DD

C

5

MCP6XXX

V

R

DD

2

MCP6XXX

C

2

V

DD

R

VDD/2

R

3

7

R

V

DD

MCP65XX

R

V

DD

MCP6XXX

8

4

V

OUT

Capacitive Sensors

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:

• RC decay

• Oscillator frequency

• Integration of current

• Wheatstone bridge

RC DECAY

The first strategy for con ditioning a capa citive sens or 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 t his time, where the thr eshold is
proportional to V
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.

PICmicro® MCU

. R1 has a low temperature

DD

R

1

P

1

P

0

C

SEN

FIGURE 17: State Variable Oscillato r.

Advantages

• Accuracy (with calibration)

• Good startup

• Easy processing using a PICmicro

®

MCU

Disadvantages

•Cost

•Design complexity

Sensor Examples

•RTD

• Hot-wire anemometer

FIGURE 18: RC Decay.

Advantages

• Ratiometric correction of VDD and temperature
errors

•Accurate

• Simple timing measurement

Disadvantages

• PICmicro MCU timing resolution

• Digital noise

• Threshold must be ratiometric

Sensor Examples

• Capacitive humidity sensor

• Capacitive touch sensor

• Capacitive tank level sensor

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AN990

OSCILLATOR FREQUENCY

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].

C

SEN

V

DD

R

2

R

3

R

V

DD

MCP65XX

R

1

V

OUT

4

FIGURE 19: Multi-vibrator (oscillator).

Advantages

•Cost

• Ratiometric operation

• Easy processing using a PICmicro

®

MCU

Disadvantages

• Reduced accuracy

Sensor Examples

• Capacitive humidity sensor

• Capacitive touch sensor

• Capacitive tank level sensor

SINGLE SLOPE INTEGRATING DETECTOR

The third strategy for conditioning a capacitive sensor
is to integrate a current and measure the elapsed time
to reach a voltage threshold. Figure20 shows a
single-slope integrating detector. Switch SW
controlled by the PICmicro
across C

at the start o f the integr atio n peri od. T he

SEN

®

MCU, zeros the voltage

voltage at the output of the op amp linearly increases
with time; the rate of increase is set by V

REF

and R1.
The comparator at the output, which can be on the
PICmicro MCU, trips at a time proportional to C

SEN

AN611 [13] discusses a similar circuit.

V

DD

V

DD

V

REF

R

C

1

SEN

to MCU

MCP65XX

MCP65XX

SW1

FIGURE 20: Single-slope Integrating
Detector.

Advantages

• Easy processing using a PICmicro® MCU

• Accuracy depends on V

REF

and R

1

Disadvantages

•Cost

Sensor Examples

• Capacitive humidity sensor

• Capacitive touch sensor

• Capacitiv e tank level sensor

,

1

.

© 2005 Microchip Technology Inc. DS00990A-page 11

AN990

CAPACITIVE WHEATSTONE BRIDGE

The fourth strategy for co nditioni ng a capa citive se nsor
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. R
that is controlled to zero-out the differential voltage, or
it can be a regular resistor. R
the instrumentation amp correctly, and to keep the
node betwee n th e capac ito rs fr om d rift ing over t ime . It
needs to be much larger than C
the divider equation can be corrected for this
resistance, if necessary.

can be a di gital potentiome ter (digi-pot)

1

provides a means to bias

3

’s impedance (1/j ωC2);

2

V

AC

C

SEN

C

2

R

3

Instrumentation

R

1

Amplifier

R

2

V

REF

V

OUT

Charge Sensors

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.

C

SEN

V

SEN

FIGURE 22: Simplified Charge Sensor
Model.

Figure 23 shows a charge amplifier (C1 and the op
amp) that co nverts the sensor e nergy (charge) to an
output voltage. R
input of the op amp, and creates a high-pass filter pol e
(keeps the inverting input of the op amp from drifting
over time). The change in charge of P
almost exclusively across C
accurate way to measure the charge produced by the
sensor.

provides a bias path for the inverting

1

appears

, which makes this an

1

R

1

SEN

FIGURE 21: Capacitive Wheatstone
Bridge.

Advantages

• Excellent common mo de noi se reje cti on

• Ratiometric output (with an ADC using V

DD

as its

reference voltage)

• Detection of open or shorted sensor (failure)

Disadvantages

• Needs AC stimulus

• Power dissipation

Sensor Examples

• Remote capacitive sensors

- Humidity sensor

- Touch sensor

- Tank level sensor

C

1

P

SEN

V

DD

R

R

V

DD

2

3

MCP6XXX

FIGURE 23: Charge Amplifier.

Advantages

• Excellent common mode noise rejection

• Ratiometric output (with an ADC using V

DD

reference voltage)

• Detection of open or shorted sensor (failure)

Disadvantages

• Needs AC stimulus

• Power dissipation

Sensor Example

• Piezo-ele ctric film

V

OUT

as its

DS00990A-page 12 © 2005 Microchip Technology Inc.

AN990

ADDITIONAL SIGNAL CONDITIONING

Circuit and firmware concerns common to many
embedded designs are mentio ned here.

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 frequ ency requirement s (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
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 so ftware design tool provi des full s chematic
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.

software [26] is

A-to-D Conversion

Many times, the conditi oned 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.

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 possib le to use a lin ear interp o­lation 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.

© 2005 Microchip Technology Inc. DS00990A-page 13

AN990

REFERENCES

General References

[1] “The OMEGA® Made in the USA Handbook™,”
Vol.1, OMEGA Engineering, Inc., 2002.

®

[2] “The OMEGA
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 Oscil la tor
Circuits For Sensor Applications,” Jim Lepkowski;
Microchip Technology Inc., DS00866, 2003.

Current Sensors

[5] AN951, “Amplifying High-Impedance Sensors –
Photodiode Example, ” Kum en Bl ake a nd 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. Ba ker; Microch ip 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.

Capacitance Sensors

Made in the USA Handbook™,”

[15] AN679, “Temperature Sensing Technologies,”
Bonnie C. Baker; Micro chip 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
Technology Inc., DS00844, 2002.

[21] AN867, “Temperature Sensing With A
Programmable Gain Amplifier,” Bonnie C. Baker;
Microchip Technology Inc., DS00867, 2003.

®

MCUs,” Joseph Julicher; Microchip

Other Sensors

[22] AN865, “Sensing Ligh t with a Prog rammable G ain
Amplifier ,” Bonnie C. Ba ker; Microch ip Technology Inc.,
DS00865, 2003.

[23] AN692, “Using a Digit al Potentiometer to Optim ize
a Precision Single-Supply Photo Detection Circuit,”
Bonnie C. Baker; Micro chip 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;” Microchip
Technology Inc., DS51419, 2003.

[27] AN942, “Piecewise Linear Interpolation on
PIC12/14/16 Series Microcontrollers,” John Day and
Steven Bible; Microchip Technology Inc., 2004.

[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
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.

• Microchip is willing to work with the customer who is concerned about the integrity of their code.

• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”

Code protection is constantly evolving. We at Microchip are com mitted to continuously improving the code protect ion f eatures of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digit al Mill ennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.

Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR WAR­RANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED,
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RELATED TO THE INFORMATION, INCLUDING BUT NOT
LIMITED TO ITS CONDITION, QUALITY, PERFOR MANCE,
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Microchip disclaims all liability arising from this information and
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Trademarks

The Microchip name and logo, the Microchip logo, Accuron,
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EELOQ, microID, MPLAB, PIC, PICmicro,

PICSTART, PRO MATE, PowerSmart, rfPIC, and
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All other trademarks mentioned herein are property of their
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© 2005, Microchip Technology Incorporated, Printed in the
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Printed on recycled paper.

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and manufacture of development systems is ISO 9001:2000 certified.

®

8-bit MCUs, KEELOQ

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code hopping

© 2005 Microchip Technology Inc. DS00990A-page 15

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