Circuit Note CN-0332 Devices Connected/Referenced 1 nV/Hz, Low Power, Rail-to-Rail Output ADA4897-2 Dual Op Amp Circuits from the Lab reference designs are engineered and Low Distortion, High Speed Rail-to-Rail AD8027 tested for quick and easy system integration to help solve todays Input/Output Op Amp analog, mixed-signal, and RF design challenges. For more Rail-to-Rail, Very Fast, 2.5 V to 5.5 V, information and/or support, visit www.analog.com/CN0332. ADCMP601 Single-Supply TTL/CMOS Comparators 50 mA, High Voltage, Micropower Linear ADP1720 Regulator Magnetoresistive (MR) Rotational Speed Sensor The complete signal conditioning solution amplifies the small EVALUATION AND DESIGN SUPPORT output voltage of the magnetoresistive sensor and converts it Circuit Evaluation Boards into a digital output signal with a rise and fall time of less than CN0332 Evaluation Board (EVAL-CN0332-PMDZ) 5 ns and an rms jitter of approximately 100 ps. Design and Integration Files Schematics, Layout Files, Bill of Materials The circuit provides a compact and cost effective robust solution for high speed rotational sensing in industrial and CIRCUIT FUNCTION AND BENEFITS automotive applications and is an excellent alternative to Hall The circuit shown in Figure 1 is a single-supply, low cost, high- effect sensors. speed magnetoresistive (MR) signal conditioner with a minimum PCB footprint. 5.3V TO 24V 5V ADP1720 IN OUT 5V 5V EN +VO1 GND 5V 1/2 R3 R4 ADA4897-2 R7 35. 7k 50 10k 5V LE/HYS 5V R1 1k V PULSE V CC1 CC2 Q OUTPUT AA747 ADCMP601 VO 1 +V +V O2 O1 AD8027 1/2 R2 ADA4897-2 1k V V R8 O2 O1 10k GND GND 5V 1 2 R5 R6 71. 5k 71. 5k MAGNETORESISTIVE SENSOR Figure 1. Magnetoresistive Speed Sensing Application Circuit (Simplified Schematic: All Connections and Decoupling 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 to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 2014 Analog Devices, Inc. All rights reserved. 10625-001CN-0332 Circuit Note The resistance of the MR element is at a maximum when is 0 . CIRCUIT DESCRIPTION The above equation suggests a non-linear performance, but it The output of the AA747 Sensitec MR sensor is from a bridge can be linearized using barber poles that provides a linear that has a typical output voltage of 30 mV p-p to 100 mV p-p at response at a certain range (see Circuit Note CN-0323, a common-mode voltage that is equal to one-half the supply Magnetoresistive Angle Measurement). voltage. The bridge output impedance is approximately 3 k. A complete MR sensor is made up of four magnetoresistive This small signal output of the sensor is first buffered by the elements arranged in a bridge configuration, resulting in a ADA4897-2 op amp. The ADA4897-2 dual op amp has a high nearly linear differential output signal. Also, the inherent bandwidth (230 MHz for G = +1) and a rail-to-rail input and temperature coefficients of the four MR elements are mutually output making it an ideal buffer for the sensor signal. Voltage compensated by the bridge configuration. noise is only 1 nV/Hz. MR sensors are good alternatives to Hall-effect sensors for low The buffered differential signal is then applied to the AD8027 magnetic field applications due to their high magnetic field op amp configured as a difference amplifier such that the gain is sensitivity. They are popular because of their non-contact operation given by: that makes them immune to wear and friction. Their rugged R3 + R4 construction provides high reliability, and they are relatively , Gain = insensitive to vibrations mechanical stress. They have a wide R1 operating frequency range (0 Hz to 1 MHz), high operating where R1 = R2 and R5 R6 = R3 + R4. temperature, low cost, small size, and fast response times. They The common-mode voltage of 2.5 V is supplied by the R5/R6 are particularly popular in rotational speed sensing because of voltage divider. their high sensitivity that can measure weak magnetic fields and The component values in the circuit are R1 = R2 = 1 k, R5 = also detect zero speed. MR sensors are therefore very popular in industrial and automotive applications. R6 = 71.5 k, R3 = 35.7 k, and R4 = 50 . These values provide a gain of 35.75. The output voltage of MR sensors are comparatively larger than Hall effect sensors, resulting to a better signal-to-noise ratio The feedback resistor is composed of R3 + R4 so that the standard resistor values can be used to create required value of 35.75 k. and EMC. Because of its higher sensitivity, a larger air gap between the MR sensor and the target wheel is allowed. Less The AD8027 is also a high bandwidth (190 MHz, G = +1) rail- strict tolerances in sensor housing and mechanical setups are to-rail amplifier with a slew rate of 100 V/s. It is stable at high possible reducing cost and complexity of the design. MR gains and is ideally suited to amplify the small signal output of sensors are also more cost effective because of the additional magnetoresistive sensors. cost of the magnets required for Hall effect sensors. The output of the AD8027 drives the ADCMP601, a very fast MR sensors are therefore very popular in rotational speed single supply rail-to-rail TTL/CMOS comparator with a propaga- sensing, especially in industrial and automotive applications. tion delay of only 3.5 ns and a minimum pulse width of 3 ns. The ADP1720 low dropout linear regulator is used to isolate the circuit from noise that can occur in harsh environments. The ADA4897-2, AD8027, ADCMP601, and ADP1720 all come in small packages, such as TSOT, MSOP, and SC-70, making the circuit well suited to applications requiring minimal PCB area. Magnetoresistive Sensors Magnetoresistance is the property of a ferromagnetic material where its resistance changes in the presence of an external magnetic field. A magnetoresistive (MR) element is produced by the deposition of a thin film of permalloy (20% Fe, 80% Ni) which gives it a preferred magnetization direction. In the presence of an external magnetic field parallel to the plane of the permalloy but perpendicular to the current flow, the internal magnetization vector of the permalloy will rotate an angle and cause a change in the resistance of the permalloy with respect to that angle. This is mathematically expressed in the equation below. R and R are properties of the permalloy, 0 and R has a typical range of 2% to 3%. 2 R = R + Rcos 0 Rev. 0 Page 2 of 6