Analog & Interface Solutions Fall 2012 Signal Chain Design Guide Devices For Use With Sensors Design ideas in this guide use the following devices. A complete device list and corresponding data sheets for these products can be found at www.microchip.com/analog.
Signal Chain Overview Typical sensor applications involve the monitoring of sensor parameters and controlling of actuators. The sensor signal chain, as shown below, consists of analog and digital domains. Typical sensors output very low amplitude analog signals. These weak analog signals are amplified and filtered, and converted to digital values using op amps, analog-to-digital or voltage-to-frequency converters, and are processed at the MCU.
Sensor Overview Many system applications require the measurement of a physical or electrical condition, or the presence or absence of a known physical, electrical or chemical quantity. Analog sensors are typically used to indicate the magnitude or change in the environmental condition, by reacting to the condition and generating a change in an electrical property as a result.
Product Overviews Operational Amplifiers (Op Amps) Microchip Technology offers a broad portfolio of op amp families built on advanced CMOS technology. These families are offered in single, dual and quad configurations, which are available in space saving packages. These op amp families include devices with Quiescent Current (Iq) per amplifier between 0.45 µA and 6 mA, with a Gain Bandwidth Product (GBWP) between 9 kHz and 60 MHz, respectively. The op amp with lowest supply voltage (Vdd) operates between 1.
Product Overviews Voltage References Digital-to-Analog Converters (DAC) Microchip offers the MCP15XX family of low power and low dropout precision Voltage References. The family includes the MCP1525 with an output voltage of 2.5V and the MCP1541 with an output voltage of 4.096V. Microchip’s voltage references are offered in SOT23-3 and TO-92 packages. Microchip’s family of Digital-to-Analog Converters (DACs) offer a wide range of options. These devices support the 6-bit through 12-bit applications.
Local Sensors Local Sensing Sensors and Applications Local sensors are located relatively close to their signal conditioning circuits, and the noise environment is not severe; most of these sensors are single ended (not differential). Non-inverting amplifiers are a good choice for amplifying most of these sensors’ output because they have high input impedance, and require a minimal amount of discrete components.
Remote Sensors Differential Amplifier All sensors in a high noise environment should be considered as remote sensors. Also, sensors not located on the same PCB as the signal conditioning circuitry are remote. Remote sensing applications typically use a differential amplifier or an instrumentation amplifier.
Weight and Pressure Sensing Applications Weight and pressure measurement have been among the most popular applications for medical, industrial, automotive and consumer industries. In recent years, the MEMS pressure/accelerometer devices have become widely used in many applications and support our modern life style. The majority of weight scale and pressure measurement circuits use bridge type ratiometric configuration.
Voltage and Current Measurement DC Voltage and Current Measurement DC voltage and current measurement can be easily done by using low speed high resolution Delta Sigma ADC such as MCP3421 and MCP3422 family devices. The MCP3421 is a single channel device while the MCP3422 is a dual channel device, which can measure the voltage and current using the same device.
Voltage and Current Measurement AC Voltage and Current Measurement Shunt resistors are a common and low cost method for current sensing. Isolated methods include the use of Current transformers and Rogowski coils. The Current Measurement using Rogowski Coil figure shows an example of the current measurement using the Rogowski coil. The Rogowski coil picks-up the electro-magnetic field (EMF) produced by the current at the center. This EMF is measured as voltage.
Temperature Sensing Solutions Resistive Temperature Detector (RTD) Solutions Thermistor Solution Thermistors are non-linear and require a look up table for compensation. The solution is to use Microchip’s Linear Active Thermistors, the MCP9700 and the MCP9701. These are low-cost voltage output temperature sensors that replace almost any Thermistor application solutions.
Temperature Sensing Solutions Resistive Temperature Detector (RTD) Solutions RTD Solution with RC Oscillators RC oscillators offer several advantages in precision sensing applications. They do not require an Analog-toDigital Converter (ADC), and oscillator can be directly connected to an Input/Output pin of a microcontroller to measure change in frequency proportional to sensor output.
Temperature Sensing Solutions Temperature Measurements Using 4 Channel ADC (MCP3424) See Thermocouple Reference Design (TMPSNSRD-TCPL1) Thermocouple Sensor Isothermal Block (Cold Junction) Isothermal Block (Cold Junction) MCP3424 Delta-Sigma ADC MCP9804 SCL SDA 0.
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 feedback capacitor (Cf) is used for circuit stability.
Sensor Calibration/Compensation Sensor Characteristics Sensor characteristics vary, both for device to device as well as for a given device over the operating conditions. To optimize system operation, this sensor variation may require some compensation. This compensation may simply address device to device variation, or be more dynamic to also address the variations of the device over the operating conditions.
Sensor Calibration/Compensation Setting the DC Set Point for Sensor Circuit A common DAC application is digitally controlling the set point and/or calibration of parameters in a signal chain. The figure below shows controlling the DC set point of a light detector sensor using the MCP4728 12-bit quad DAC device. The DAC provides 4096 output steps. If G = 1 and internal reference voltage options are selected, then the internal 2.048 Vref would produce 500 µV of resolution.
Oscillator Circuits For Sensors RC oscillators can accurately and quickly measure resistive and capacitive sensors. The oscillator period (or frequency) is measured against a reference clock signal, so no analog-to-digital convertor is needed.
Development Software FilterLab® Software Microchip’s FilterLab software is an innovative software tool that simplifies analog active filter (using op amps) design. Available at no cost from the Microchip website at www.microchip.com/filterlab, the FilterLab design tool provides full schematic diagrams of the filter circuit with component values. It also outputs the filter circuit in SPICE format, which can be used with the macro model to simulate actual filter performance.
Development Tools These following development boards support the development of signal chain applications. These product families may have other demonstration and evaluation boards that may also be useful. For more information visit www.microchip.com/analogtools. Reference Designs Battery MCP3421 Battery Fuel Gauge Demo (MCP3421DM-BFG) The MCP3421 Battery Fuel Gauge Demo Board demonstrates how to measure the battery voltage and discharging current using the MCP3421.
Development Tools Demonstration Boards ADCs MCP3911 ADC Evaluation Board for 16-bit MCUs (ADM00398) The MCP3911 ADC Evaluation Board for 16-Bit MCUs system provides the ability to evaluate the performance of the MCP3911 dual-channel ADC. It also provides a development platform for 16-bit PIC-based applications, using existing 100-pin PIM systems compatible with the Explorer-16 and other high pin count PIC demo boards.
Development Tools MCP6V01 Input Offset Demo Board (MCP6V01DM-VOS) MCP6SX2 PGA Thermistor PICtail Demo Board (MCP6SX2DM-PCTLTH) The MCP6V01 Input Offset Demo Board is intended to provide a simple means to measure the MCP6V01/2/3 op amps input offset voltage (Vos) under a variety of bias conditions. This Vos includes the specified input offset voltage value found in the data sheet plus changes due to power supply voltage (PSRR), common mode voltage (CMRR), output voltage (AOL) and temperature (IVos/ITA).
Related Support Material The following literature is available on the Microchip web site: www.microchip.com/appnotes. There are additional application notes that may be useful. Application Related Documentation Sensor Conditioning Circuits Overview AN866: Designing Operational Amplifier Oscillator Circuits For Sensor Applications Operational amplifier (op amp) oscillators can be used to accurately measure resistive and capacitive sensors.
Related Support Material Op Amps AN1302: Current Sensing Circuit Concepts and Fundamentals This application note provides an overview of current sensing circuit concepts and fundamentals. It introduces current sensing techniques and focuses on three typical high-side current sensing implementations, with their specific advantages and disadvantages.
Related Support Material AN1297: Microchip’s Op Amp SPICE Macro Models This application note covers the function and use of Microchip’s op amp SPICE macro models. It does not explain how to use the circuit simulator but will give the user a better understanding how the model behaves and tips on convergence issues. AN1353: Rectifiers, Op Amp Peak Detectors and Clamps This application note covers a wide range of application, such as half-wave rectifiers, full-wave rectifiers, peak detectors and clamps.
Related Support Material Passive Keyless Entry (PKE) TB090: MCP2030 Three-Channel Analog Front-End Device Overview Temperature Sensing AN981: Interfacing a MCP9700 Analog Temperature Sensor to a PIC Microcontroller This tech brief summarizes the technical features of the MCP2030 and describes how the three channel standalone analog front-end device can be used for various bidirectional communication applications.
linear Linear: Op Amps Signal Chain Design Guide # per Package GBWP (kHz) Typ. Iq (µA/amp.) Typ. Vos (±µV) Max. Supply Voltage (V) Temperature Range (°C) Railto-Rail I/O 1, 2, 4 9 0.45 4,500 1.8 to 6.0 −40 to +125 I/O Low Quiescent Current SOIC, MSOP, 2 × 3 TDFN, TSSOP, SOT-23, SC-70 SOIC8EV, SOIC14EV – MCP6031/2/3/4 1, 2, 1, 4 10 1 150 1.8 to 5.
Linear: Op Amps (Continued) Device # per Package GBWP (kHz) Typ. Iq (µA/amp.) Typ. Vos (±µV) Max. Supply Voltage (V) Temperature Range (°C) Railto-Rail I/O Features MCP621/1S/2/3/4/5/9 1, 2, 3, 1, 4, 2, 4 20,000 2,500 200 2.5 to 5.5 −40 to +125 O mCal (offset correction), Low Power Mode on MCP623/5/9 SOT-23, SOIC, MSOP, DFN, TSSOP, QFN MCP651EV-VOS High Speed, High Output Drive, Low Offset MCP631/2/3/4/5/9 1, 2, 1, 4, 2, 4 24,000 2,500 8,000 2.5 to 5.
Linear: Programmable Gain Amplifiers (PGA) Device −3 dB BW (MHz) Typ. Channels Signal Chain Design Guide MCP6S21/2/6/8 Iq (µA) Max. Vos (±µV) Max. Operating Voltage (V) Temperature Range (°C) 1, 2, 6, 8 2 to 12 1.1 275 2.5 to 5.5 −40 to +85 1, 2, 2 1 to 18 1.0 4000 2.5 to 5.
Mixed Signal: Successive Approximation Register (SAR) A/D Converters Part # MCP3001 MCP3002 Resolution (bits) Max.Sample Rate (samples/sec) # of Input Channels Input Type Interface Input Voltage Range (V) Max. Supply Current (µA) Max. INL Temperature Range (°C) 10 200 1 Single−ended SPI 2.7 to 5.5 500 ±1 LSB −40 to +85 PDIP−8, SOIC−8, MSOP−8, TSSOP−8 – −40 to +85 PDIP−8, SOIC−8, MSOP−8, TSSOP−8 – 10 200 2 Single−ended SPI 2.7 to 5.
Mixed Signal: D/A Converters (Continued) Signal Chain Design Guide Resolution (Bits) DACs per Package Interface Vref Output Settling Time (µs) DNL (LSB) Typical Standby Current (µA) Typical Operating Current (µA) Temperature Range (°C) MCP4911 10 1, 2 SPI Ext 4.5 0.5 1 175 −40 to +125 PDIP-8, SOIC-8, MSOP-8, PDIP-14, SOIC-14, TSSOP-14 – MCP4912 10 1, 2 SPI Ext 4.5 0.5 1 350 −40 to +125 PDIP-8, SOIC-8, MSOP-8, PDIP-14, SOIC-14, TSSOP-14 – MCP4921 12 1 SPI Ext 4.
Mixed Signal: Digital Potentiometers (Continued) Device # of Taps # per Package Interface Vdd Operating Range(1) Volatile/ Non-Volatile Resistance (Ω) INL (Max.) DNL (Max.) Temperature Range (°C) MCP4151 257 1 SPI 1.8V to 5.5V Volatile 5K, 10K, 50K, 100K ±1 LSb ±0.5 LSb −40 to +125 PDIP-8, SOIC-8, MSOP-8, DFN-8 MCP42XXDM-PTPLS MCP4152 257 1 SPI 1.8V to 5.5V Volatile 5K, 10K, 50K, 100K ±1 LSb ±0.
Thermal management Thermal Management: Temperature Sensors Signal Chain Design Guide Part # Typical Accuracy (°C) Maximum Accuracy @ 25 (°C) Maximum Temperature Range (°C) Vcc Range (V) Maximum Supply Current (µA) Resolution (bits) Packages Featured Demo Board Serial Output Temperature Sensors MCP9800 ±0.5 ±1 −55 to +125 2.7 to 5.5 400 9-12 SOT-23-5 MCP9800DM-TS1 MCP9801 ±0.5 ±1 −55 to +125 2.7 to 5.5 400 9-12 SOIC-8 150 mil, MSOP-8 MCP9800DM-TS1 MCP9802 ±0.
Stand-Alone Analog and Interface Portfolio Thermal Management Power Management Temperature Sensors LDO & Switching Regulators Fan Speed Controllers/ Fan Fault Detectors Charge Pump DC/DC Converters Motor Drivers Power MOSFET Drivers PWM Controllers Stepper and DC System Supervisors 3-Phase Brushless DC Fan Controller Voltage References Voltage Detectors Li-Ion/Li-Polymer Battery Chargers Linear Op Amps Instrumentation Amplifiers Programmable Gain Amplifiers Comparators Safety & Security Photoe
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