AN990 Analog Sensor Conditioning Circuits – An Overview Author: Kumen Blake In addition, circuit and firmware concerns common to Microchip Technology Inc. many embedded designs are briefly mentioned: • Input Protection INTRODUCTION • Sensor Failure Detection •Filtering Target Audience • Analog-to-Digital (A-to-D) Conversion • Correction of Results This application note is intended for hardware design References to documents that treat these subjects in engineers that need to condition the output of common more depth have been included in the “References” analog sensors. section. Goals SENSOR APPLICATIONS • Review sensor applications (e.g., temperature) • Review sensor types (e.g., voltage output) This section reviews a few analog sensor applications. For each application, a list of common sensor types is • Show various conditioning circuits given for convenience. A good resource for many of • Give technical references these applications is OMEGA® Engineering’s handbooks [1, 2]. Description There are many more analog sensors than the ones Analog sensors produce a change in an electrical discussed in this application note. For example: property to indicate a change in its environment. This • Time/frequency counters [14] change in electrical property needs to be conditioned • Distance ranging sensor [25] by an analog circuit before conversion to digital. Further processing occurs in the digital domain but is • Current sensing transformer [6] not addressed in this application note. Emphasis is placed on the electrical behavior of the The applications mentioned are: various sensors. It is necessary to know this information when selecting an appropriate sensor • Electrical conditioning circuit. • Magnetic • Temperature Electrical • Humidity These applications measure the state at some point in • Force, Weight, Torque and Pressure an electrical circuit. They include monitoring the • Motion and Vibration condition of a crucial electrical circuit or power source. •Flow • Fluid Level and Volume TABLE 1: ELECTRICAL APPLICATIONS • Light and Infrared (IR) Sensor Electrical Parameter •Chemistry Voltage Voltage For each type of electrical property, commonly used Current Current conditioning circuits are shown. Each circuit has an accompanying list of advantages and disadvantages, Charge Charge and a list of sensor types appropriate for that circuit. The electrical properties covered are: •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 Some common analog motion and vibration sensors strength and/or direction. They are commonly used in are listed in Table 6. In many cases, more integrated compasses and motor control [6]. solutions are available. TABLE 2: MAGNETIC APPLICATIONS TABLE 6: MOTION AND VIBRATION Sensor Electrical Parameter APPLICATIONS Hall effect [6] Voltage Sensor Electrical Parameter Magneto-resistive Resistance LVDT [10] AC Voltage Piezo-electric Voltage or Charge Temperature Microphone Voltage The most common sensor application is temperature Motor Sensors [6] Voltage, Resistance, measurement. Some common sensors are listed in Current, ... Table 3. Overviews of temperature sensors can be Ultrasonic Distance [25] Time found in the references [14, 15]. IC Accelerometers Voltage TABLE 3: TEMPERATURE Flow APPLICATIONS Many different approaches are used for measuring the Sensor Electrical Parameter flow of liquids and gases. A short sample is shown in Thermocouple [19, 20] Voltage Table 7. RTD [18] Resistance Thermistor [16, 17] Resistance TABLE 7: FLOW APPLICATIONS IC Voltage Sensor Electrical Parameter IR Thermal Sensor Current Magnetic Flow Meter AC Voltage Thermo Piles Voltage Mass Flow Meter Resistance (temperature) Humidity Ultrasound/Doppler Frequency Two common ways to measure humidity are listed in Hot-wire Anemometer Resistance Table 4. It is often necessary to compensate for [24] temperature in these applications. Mechanical Transducer Voltage, ... (e.g., turbine) TABLE 4: HUMIDITY APPLICATIONS Sensor Electrical Parameter Fluid Level and Volume Capacitive Capacitance Table 8 gives several examples of fluid level sensors. Fluid volume in a rigid container can be calculated from Infrared (IR) Current the level. Force, Weight, Torque, and Pressure TABLE 8: FLUID LEVEL AND VOLUME The sensors in this section measure a mechanical APPLICATIONS force or strain. Common types are listed in Table 5. Sensor Electrical Parameter TABLE 5: FORCE, WEIGHT, TORQUE, Ultrasound Time AND PRESSURE Mechanical Transducer Resistance, Voltage, ... APPLICATIONS Capacitive Capacitance Sensor Electrical Parameter Switch (e.g., vibrating) On/Off Thermal — Strain Gage [8 - 10] Resistance Load Cell Resistance Piezo-electric Voltage or Charge Mechanical Transducer Resistance, Voltage, ... DS00990A-page 2 © 2005 Microchip Technology Inc. AN990 Light and Infrared (IR) Advantages Light and IR are used to detect the presence of objects • High input impedance (e.g., people in a burglar alarm) and reduction in • Low bias current (CMOS op amps) visibility (smoke and turbidity detectors). • Positive gain • Simplicity TABLE 9: LIGHT AND IR APPLICATIONS Disadvantages Sensor Electrical Parameter • Limited input voltage range • Input stage distortion Photodiode [22, 23] Current • Amplifies common mode noise Chemistry Sensor Examples Table 10 gives a short list of sensors that detect • Thermocouple chemical conditions. • Thermo pile • Piezo-electric film TABLE 10: CHEMISTRY APPLICATIONS Sensor Electrical Parameter BUFFER FOR HIGH IMPEDANCE VOLTAGE SOURCE pH Electrode Voltage (with high output impedance) This circuit requires a FET input op amp (e.g., CMOS input); see Figure 2. The FET input gives very high Solution Conductivity Resistance input impedance and very low input bias current, espe- CO Sensor Voltage or Charge cially at room temperature (the ESD diodes conduct Turbidity (photodiode) Current more current at higher temperatures). The operational Colorimeter (photodiode) Current amplifier (op amp) is used as a non-inverting amplifier. BASIC SIGNAL CONDITIONING VDD CIRCUITS VSEN MCP6XXX VOUT 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 R R circuit. 1 2 FET Input Op Amp Voltage Sensors The circuits in this section condition a voltage produced FIGURE 2: Non-inverting Gain Amplifier by a sensor. for High-Impedance Sensors with Voltage Output. NON-INVERTING GAIN AMPLIFIER Advantages Figure 1 shows a non-inverting gain amplifier using an • Very high input impedance op amp. It presents a high impedance to the sensor (at • Very low bias current (CMOS op amps) VSEN) and produces a positive gain from VSEN to VOUT. • Positive gain • Simplicity VDD Disadvantages V MCP6XXX SEN • Limited input voltage range VOUT • Input stage distortion • Amplifies common mode noise Sensor Example R1 R2 • 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 temper- Advantages ature and can be quite large. Its output voltage is • Resistive isolation from the source proportional to absolute temperature. • Large input voltage range is possible INVERTING GAIN AMPLIFIER • Rejects common mode noise; it is good for remote sensors Figure 3 shows an inverting gain amplifier using an op • Simplicity amp. It presents an impedance of R1 to the sensor (at V ) and produces a negative gain from V to SEN SEN Disadvantages VOUT. • Resistive loading of the source VDD • Input stage distortion MCP6XXX Sensor Examples V OUT • Remote thermocouple • Wheatstone bridge VSEN INSTRUMENTATION AMPLIFIER R1 R2 Figure 5 shows an instrumentation amplifier circuit that FIGURE 3: Inverting Gain Amplifier. conditions a remote voltage sensor. The input resistors provide isolation and detection of sensor open-circuit Advantages failure. It amplifies the input difference voltage (V +–V –) and rejects common mode noise. • Resistive isolation from the source SEN SEN • Large input voltage range is possible VDD • Virtually no input stage distortion • Simplicity VDD R1 R Instrumentation Disadvantages 2 VSEN+ Amplifier • Resistive loading of the source VOUT V – V • Inverting gain SEN REF R2 • Amplifies common mode noise R1 Sensor Examples • Thermo pile FIGURE 5: Instrumentation Amplifier. • High-side (VDD) voltage sensor Advantages DIFFERENCE AMPLIFIER • Excellent rejection of common mode noise; it is Figure 4 shows a difference amplifier using an op amp. great for remote sensors It presents an impedance of R1 to each end of the • Resistive isolation from the source sensor (V + and V –) and amplifies the input SEN SEN • Detection of sensor failure difference voltage (VSEN+–VSEN–). Disadvantages R R 1 2 • Resistive loading of the source V + SEN •Cost VDD Sensor Examples MCP6XXX • Remote thermocouple VOUT • Remote RTD (with a current source or voltage divider to produce a voltage from the RTD) • Wheatstone bridge VSEN– R1 R2 - Strain gage - Pressure sensor FIGURE 4: Difference Amp. DS00990A-page 4 © 2005 Microchip Technology Inc. AN990 VARIABLE GAIN FOR WIDE DYNAMIC RANGE Current Sensors AND NON-LINEAR SENSORS The circuits in this section condition a current produced Figure 6 shows a Programmable Gain Amplifier (PGA) by a sensor. used to condition multiple sensors. These PGAs (e.g., MCP6S22) allow the user to select an input