Circuit Note CN-0288 Devices Connected/Referenced AD598 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/CN0288. 2 2-Channel, 12-Bit ADC with I C Compatible AD7992 Interface in 10-lead MSOP LVDT Signal Conditioning Circuit This circuit uses the AD598 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-0288 Circuit Evaluation Board (EVAL-CN0288-SDPZ) AD598 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 AD598 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, making this ideal for remote applications. The circuit can operate a remote LVDT from up to 300 feet away, and the output The LVDT is a highly reliable sensor because the magnetic core can drive up to 1000 feet. can move without friction and does not touch the inside of the tube. Therefore, LVDTs are suitable for flight control feedback This circuit note discusses basic LVDT theory of operation and the systems, position feedback in servomechanisms, automated design steps used to optimize the circuit shown in Figure 1 for a measurement in machine tools, and many other industrial and chosen bandwidth, including noise analysis and component scientific electromechanical applications where long term reliability selection considerations. is important. +15V EXCITATION (CARRIER) 3 2 20 V A 11 +5V AMP OSC +5V AD598 17 SDA 33 V 1 SCL AD7992 IN V OUT 3k A B 16 FILTER AMP ALERT V AD8615 B A + B 2.7nF 10 0.01F 1 E-100 ECONOMY SERIES LVDT 15V Figure 1. LVDT Signal Conditioning Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. A Circuits from the Lab circuits 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 determining its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause Fax: 781.461.3113 20132014 Analog Devices, Inc. All rights reserved. whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 11426-001CN-0288 Circuit Note The ratiometric principle upon which the AD598 operates CIRCUIT DESCRIPTION requires that the sum of the LVDT secondary voltages remains Theory of Operation constant with LVDT stroke length. Although LVDT manufacturers An LVDT is an absolute displacement transducer that converts generally do not specify the relationship between V A + VB and a linear displacement or position from a mechanical reference stroke length, it is recognized that some LVDTs do not meet this (or zero) into a proportional electrical signal containing phase requirement. In these cases, a nonlinearity results. However, the (for direction) and amplitude information (for distance). The majority of available LVDTs do in fact meet these requirements. LVDT operation does not require electrical contact between the Component Selection moving part (probe or core rod assembly) and the transformer. Instead, it relies on electromagnetic coupling. For this reason, The design procedure for the dual supply operation (15 V) and because they operate without any built-in electronic circuitry, found in the AD598 data sheet was followed to set the excitation LVDTs are widely used in applications where long life and high frequency to 2.5 kHz, system bandwidth to 250 Hz, and an reliability under severe environments are a required, such output voltage from 0 V to 5 V. military and aerospace applications. It is normal for the AD598 internal oscillator to produce a small For this circuit, the E-100 Economy Series LVDT sensor from amount of ripple that feeds through to the output. A passive Measurement Specialties, Inc. was used with the AD598. With low-pass filter is used to reduce this ripple to the required level. a linearity of 0.5% of full range, the E Series is suitable for When selecting capacitor values to set the bandwidth of the system, most applications with moderate operation temperature a trade-off is involved. Choosing smaller capacitors give higher environments. system bandwidth but increase the amount of output voltage The AD598 is a complete, LVDT signal conditioning subsystem. ripple. The ripple can be reduced by increasing the shunt It converts the transducer mechanical position of LVDTs to a capacitance across the feedback resistor used to set the output unipolar dc voltage with a high degree of accuracy and voltage level however, this also increases phase lag. repeatability. All circuit functions are included on the chip. The AD8615 operational amplifier buffers the output of the With the addition of a few external passives components to AD598, which ensures that the AD7992 ADC is driven by a low set frequency and gain, the AD598 converts the raw LVDT impedance source (high source impedances significantly affect secondary output to a scaled dc signal. the ac performance of the ADC). The AD598 contains a low distortion sine wave oscillator to The low-pass filter between the output of the AD598 and the drive the LVDT primary. The frequency of the sine wave is input of the AD8615 serves two purposes: determined by a single capacitor and can range from 20 Hz to It limits the input current to the AD8615 20 kHz with amplitudes from 2 V rms to 20 V rms. It filters the output voltage ripple. The LVDT secondary output consists of two sine waves that drive the AD598 directly. The AD598 operates upon the two signals, The AD8615 has internal protective circuitry that allows voltages dividing their difference by their sum and producing a scaled exceeding the supply to be applied at the input. This is important unipolar dc output. Previous LVDT conditioners synchronously because the output voltage of the AD598 can swing 11 V with detect this amplitude difference and convert its absolute value to 15 V supplies. As long as the input current is limited to less a voltage proportional to position. This technique uses the primary than 5 mA, higher voltages can be applied to the input. This is excitation voltage as a phase reference to determine the polarity primarily due to the extremely low input bias current of the of the output voltage. There are a number of problems associated AD8615 (1 pA) which allows the use of larger resistors. The with this technique. They include: use of these resistors adds thermal noise, which contributes to the overall output voltage noise of the amplifier. Producing a constant amplitude, constant frequency The AD8615 is an ideal amplifier to buffer and drive the input excitation signal of the AD7992 12-bit SAR ADC because of its input overvoltage Compensating for LVDT primary to secondary phase shifts protection, and its ability to swing rail-to-rail at both the input Compensating for these shifts as a function of temperature and output. and frequency The AD598 eliminates all of these problems. The AD598 does not require a constant amplitude because it works on the ratio of the difference and sum of the LVDT output signals. A constant frequency signal is not necessary because the inputs are rectified and only the sine wave carrier magnitude is processed. There is no sensitivity to phase shift between the primary and the LVDT outputs because synchronous detection is not employed. Rev. A Page 2 of 6