Circuit Note CN-0301 Devices Connected/Referenced AD698 Universal LVDT Signal Conditioner Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve todays Precision, 20 MHz, CMOS, Single RRIO AD8615 analog, mixed-signal, and RF design challenges. For more Operational Amplifier information and/or support, visit www.analog.com/CN0301. 2 2-Channel, 12-Bit ADC with I C-Compatible AD7992 Interface in 10-Lead MSOP Universal LVDT Signal Conditioning Circuit This circuit uses the AD698 LVDT signal conditioner that contains EVALUATION AND DESIGN SUPPORT a sine wave oscillator and a power amplifier to generate the Circuit Evaluation Boards excitation signals that drive the primary side of the LVDT. The CN-0301 Circuit Evaluation Board (EVAL-CN0301-SDPZ) AD698 also converts the secondary output into a dc voltage. The System Demonstration Platform (EVAL-SDP-CB1Z) AD8615 rail-to-rail amplifier buffers the output of the AD698 and Design and Integration Files drives a low power 12-bit successive approximation analog-to- Schematics, Layout Files, Bill of Materials digital converter (ADC). The system has a dynamic range of 82 dB CIRCUIT FUNCTION AND BENEFITS and a system bandwidth of 250 Hz, making it ideal for precision industrial position and gauging applications. The circuit shown in Figure 1 is a complete adjustment-free linear variable differential transformer (LVDT) signal conditioning The signal conditioning circuitry of the system consumes only circuit. This circuit can accurately measure linear displacement 15 mA of current from the 15 V supply and 3 mA from the +5 V (position). supply. The LVDT is a highly reliable sensor because the magnetic core This circuit note discusses basic LVDT theory of operation and the can move without friction and does not touch the inside of the design steps used to optimize the circuit shown in Figure 1 for a tube. Therefore, LVDTs are suitable for flight control feedback chosen bandwidth, including noise analysis and component systems, position feedback in servomechanisms, automated selection considerations. measurement in machine tools, and many other industrial and scientific electromechanical applications where long term reliability is important. +15V EXCITATION (CARRIER) +5V VOLTAGE AMP REFERENCE OSC +5V AD698 B SDA 33 AD7992 3k V 1 SCL V IN OUT A AMP AD8615 B ALERT LPF 0.01F 2.7nF A 15V E-100 ECONOMY SERIES LVDT Figure 1. Universal LVDT Signal Conditioning Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. A 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 20132014 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) 11607-001CN-0301 Circuit Note The block diagram of the AD698 is depicted Figure 2. The CIRCUIT DESCRIPTION inputs consist of two independent synchronous demodulation Theory of Operation channels. The B channel monitors the drive excitation to the An LVDT is an absolute displacement transducer that converts LVDT. The full wave rectified output is filtered by C2 before a linear displacement or position from a mechanical reference being sent to the computational circuit. Channel A is identical (or zero) into a proportional electrical signal containing phase except that the comparators pins are externally available. Since (for direction) and amplitude information (for distance). The the A channel may reach 0 V output at the LVDT null, the A LVDT operation does not require electrical contact between the channel demodulator is usually triggered by the primary voltage moving part (probe or core rod assembly) and the transformer. (B Channel). In addition, a phase compensation network may Instead, it relies on electromagnetic coupling. For this reason, be required to add phase lead or lag to the A Channel to and because they operate without any built-in electronic circuitry, compensate for the LVDT primary to secondary phase shift. For LVDTs are widely used in applications where long life and high half-bridge circuits the phase shift in noncritical, and the A reliability under severe environments are a required, such channel voltage is large enough to trigger the demodulator. military and aerospace applications. C2 +V For this circuit, the E-100 Economy Series LVDT sensor from S BFILT1 BFILT2 Measurement Specialties, Inc. was used with the AD698. With a linearity of 0.5% of full range, the E Series is suitable for most B C5 CHANNEL applications with moderate operation temperature environments. BIN R2 1 V/I C4 The AD698 is a complete, LVDT signal conditioning subsystem. OUT +BIN LPF FILTER FB V It converts the transducer mechanical position of LVDTs to a OUT unipolar dc voltage with a high degree of accuracy and COMP repeatability. All circuit functions are included on the chip. DUTY CYCLE DIVIDER A With the addition of a few external passives components to B A/B = 1 = 100% A set frequency and gain, the AD698 converts the raw LVDT CHANNEL DUTY ACOMP secondary output to a scaled dc signal. COMP +ACOMP The AD698 contains a low distortion sine wave oscillator to drive OFF2 OFF1 IREF AIN V the LVDT primary. The frequency of the sine wave is determined 500A 1 V/I by a single capacitor and can range from 20 Hz to 20 kHz with LPF +AIN AD698 amplitudes from 2 V rms to 24 V rms. DEMODULATOR The LVDT secondary output consists of two sine waves that drive AFILT1 AFILT2 V S the AD698 directly. The AD698 decodes LVDTs by synchronously C3 demodulating the amplitude modulated input (secondaries), A, Figure 2. AD698 Block Diagram and a fixed input reference (primary or sum of secondaries or Once both channels are demodulated and filtered a division fixed input), B. A common problem with earlier solutions was circuit, implemented with a duty cycle multiplier, is used to that any drift in the amplitude of the drive oscillator corresponded calculate the ratio A/B. The output of the divider is a duty cycle. directly to a gain error in the output. The AD698 eliminates When A/B is equal to 1, the duty cycle will be equal to 100%. these errors by calculating the ratio of the LVDT output to its (This signal can be used as is if a pulse width modulated output input excitation in order to cancel out any drift effects. This is required.) The duty cycle drives a circuit that modulates and device differs from the AD598 LVDT signal conditioner in that filters a reference current proportional to the duty cycle. The it implements a different circuit transfer function and does not output amplifier scales the 500A reference current converting require the sum of the LVDT secondaries (A + B) to be constant to a voltage. The output transfer function is thus: with stroke length. A V = I R 2 OUT REF B where I = 500 A. REF Rev. A Page 2 of 7 11607-002