Circuit Note CN-0341 Devices Connected/Referenced Dual Channel, 1MSPS, 12-bit, Circuits from the Lab reference designs are engineered and AD7866 Simultaneous Sampling SAR ADC tested for quick and easy system integration to help solve todays analog, mixed-signal, and RF design challenges. For more Wide Supply Range, Rail-to-Rail, AD8227 information and/or support, visit www.analog.com/CN0341. Instrumentation Amplifier AD8615 Low Offset, Low Noise, Precision Amplifier Magnetoresistive Linear Position Measurement The circuit provides all necessary signal conditioning including EVALUATION AND DESIGN SUPPORT instrumentation amplifiers, buffers, and a dual channel ADC Circuit Evaluation Boards that efficiently process the AMR sensor low level bridge outputs. CN-0341 Circuit Evaluation Board (EVAL-CN0341-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) The result is an industry leading position measurement solution Design and Integration Files suitable for valve and flow measurement, machine tool speed Schematics, Layout Files, Bill of Materials control, motor speed measurement, and other industrial or automotive applications. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 provides a contactless, AMR (anisotropic magnetoresistive) linear position measurement solution with 2 mil (0.002 inch) accuracy over a 0.5 inch range. The circuit is ideal for applications where high speed, accurate, non-contact length and position measurements are critical. 5V 0.1F 10F 5V 5V 2.5V 3.3V 5V REF DV AV REFSEL RANGE SDP DD DD 0.1F 10F 2.96k AD8227 V V AD8615 A1 DRIVE V CC SCLK +V O1 AD7866 5V 2.5V D A OUT V O1 AA745 +V 5V O2 CS REF V O2 2.96k AD8227 GND1 V AD8615 B1 DGND AGND D AD B V CAP CAP REF 5V 470nF 470nF 100nF 2.5V AD8615 Figure 1. Magnetoresistive Linear Position Measurement System (Simplified Schematic: Decoupling and All Connections Not Shown) Rev. 0 Circuits from the Lab reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 2014 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 12115-001CN-0341 Circuit Note An example of the AMR effect is shown in Figure 2. A current (I), CIRCUIT DESCRIPTION flowing through a conductor, is subject to an external magnetic The Sensitec AA745 is an AMR-based angular sensor containing field (H ). The resistance of the conductor changes as a function of Y two galvanically separated Wheatstone bridges at a relative angle of the angle () between the magnetization vector (M) and the 45 to each other. The AA745 offers minimal offset voltage current flow vector (I). The magnetization vector is the net sum (2 mV) and high signal amplitude (70 mV). A rotating magnetic of the internal magnetic field (H ) and the applied external X field stimulates the sensor, creating an output voltage of 70 mV. magnetic field (H ). Y An AD8227 instrumentation amplifier amplifies the signal of The maximum resistance occurs when the magnetization vector interest while rejecting the Wheatstone bridge common-mode (M) is parallel to the current vector (I). The minimum resistance voltage of 2.5 V. The common-mode output voltage of the in- occurs when the magnetization vector (M) is perpendicular to amp is set to 2.5 V by driving the VREF pin to 2.5 V. A 2.96 k the current vector (I). gain resistor sets the gain of 32. This creates an analog output voltage of 0.2 V to 4.8 V for a bridge output of 2.5 V 70 mV. Effective utilization of the AMR effect requires the conductor to be a material insensitive to mechanical stress but sensitive to magneto- The circuit signal bandwidth is determined by the AD8227 that restriction. For these reasons, permalloy (80% nickel, 20% iron) has an approximate 100 kHz bandwidth for a gain of 32. is the most commonly used alloy in AMR sensor manufacturing. A unity gain AD8615 op amp buffers the in-amp output voltage Permalloy Properties and connects directly to the ADC. This buffer has a rail-to-rail There are two properties of permalloy strips that provide design output stage that swings to within 200 mV of the supply rails. challenges when creating angular measurement systems. The AD7866 is a dual channel 12-bit 1 MSPS SAR ADC. The First, permalloy has a narrow linear operating region (see Figure 3). polarity of the RANGE pin determines the analog input range Only when the angle () between the magnetization vector (M) and output coding. If this pin is tied to a logic high when the chip and current flow vector (I) becomes larger, does the response select goes low, the analog input range of the next conversion is 0 V to 2V (0 V to 5 V), leaving approximately 200 mV headroom become linear. Unfortunately, shortly after the response becomes REF linear, it saturates. for the 0.2 V to 4.8 V input signal from the buffer amplifier. R Connecting the REFSEL pin low configures the ADC to use the internal 2.5 V reference voltage. This voltage is available on the R + R 0 VREF pin but requires a buffer before it can be used elsewhere in the system. The D A pin and D B pin are decoupled with CAP CAP 470 nF capacitors to ensure proper operation of the ADC. The reference voltage is buffered by the AD8615 and sets the common-mode output voltage of the AD8227 in-amp. R 0 The AD7866 simultaneously samples both channels of the H Y magnetoresistive sensor. The digital words are normally available H 0 1.0 0.5 0 0.5 1.0 on DOUTA and DOUTB. Each data stream consists of one leading Figure 3. Permalloy Resistance vs. Magnetic Field zero followed by three status bits and then twelve bits of conversion data. However, by holding the chip select low for an additional 16 Secondly, permalloy is insensitive to polarity. The resistance of a clock cycles, both digital words are read from one channel, D A. OUT permalloy strip decreases whether the angle () between the magnetization vector (M) and the current flow vector (I) is An SPI interface allows access to both channels on one data line. positive or negative. Magnetoresistive (MR) Theory Barber Poles Magnetoresistivity is the ability of a material to change the value A common method used to improve both the linearity and polar of its resistance when subjected to an external magnetic field. insensitivity of the permalloy strip is to add aluminum stripes The most commonly used MR sensors are based on the angled at 45 to the strip axis called barber poles, as shown in anisotropic magnetoresistive (AMR) effect. Figure 4. Any current flowing between barber poles takes the M shortest paththe perpendicular path, and the angle between the current flow vector (I) and magnetization vector (M) shifts by 45. I H Y ALUMINUM STRIPES H Y I H X M Figure 2. Anisotropic Magnetoresistive Example H X H = 0 Y Figure 4. Barber Pole Effect in a Permalloy Strip Rev. 0 Page 2 of 8 12115-002 12115-004 12115-003