CN0288

Theory of Operation

An LVDT is an absolute displacement transducer that converts a linear displacement or position from a mechanical reference (or zero) into a proportional electrical signal containing phase (for direction) and amplitude information (for distance). The LVDT operation does not require electrical contact between the moving part (probe or core rod assembly) and the transformer. Instead, it relies on electromagnetic coupling. For this reason, and because they operate without any built-in electronic circuitry, LVDTs are widely used in applications where long life and high reliability under severe environments are a required, such military and aerospace applications.

For this circuit, the E-100 Economy Series LVDT sensor from Measurement Specialties™, Inc. was used with the AD598. With a linearity of ±0.5% of full range, the E Series is suitable for most applications with moderate operation temperature environments.

The AD598 is a complete, LVDT signal conditioning subsystem. It converts the transducer mechanical position of LVDTs to a unipolar dc voltage with a high degree of accuracy and repeatability. All circuit functions are included on the chip. With the addition of a few external passives components to set frequency and gain, the AD598 converts the raw LVDT secondary output to a scaled dc signal.

The AD598 contains a low distortion sine wave oscillator to drive the LVDT primary. The frequency of the sine wave is determined by a single capacitor and can range from 20 Hz to 20 kHz with amplitudes from 2 V rms to 20 V rms.

The LVDT secondary output consists of two sine waves that drive the AD598 directly. The AD598 operates upon the two signals, dividing their difference by their sum and producing a scaled unipolar dc output. Previous LVDT conditioners synchronously detect this amplitude difference and convert its absolute value to a voltage proportional to position. This technique uses the primary excitation voltage as a phase reference to determine the polarity of the output voltage. There are a number of problems associated with this technique. They include:

• Producing a constant amplitude, constant frequency excitation signal
• Compensating for LVDT primary to secondary phase shifts
• Compensating for these shifts as a function of temperature 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.

The ratiometric principle upon which the AD598 operates requires that the sum of the LVDT secondary voltages remains constant with LVDT stroke length. Although LVDT manufacturers generally do not specify the relationship between V_A + V_B and stroke length, it is recognized that some LVDTs do not meet this requirement. In these cases, a nonlinearity results. However, the majority of available LVDTs do in fact meet these requirements.

Component Selection

The design procedure for the dual supply operation (±15 V) found in the AD598 data sheet was followed to set the excitation frequency to 2.5 kHz, system bandwidth to 250 Hz, and an output voltage from 0 V to 5 V.

It is normal for the AD598 internal oscillator to produce a small amount of ripple that feeds through to the output. A passive low-pass filter is used to reduce this ripple to the required level.

When selecting capacitor values to set the bandwidth of the system, a trade-off is involved. Choosing smaller capacitors give higher system bandwidth but increase the amount of output voltage ripple. The ripple can be reduced by increasing the shunt capacitance across the feedback resistor used to set the output voltage level; however, this also increases phase lag.

The AD8615 operational amplifier buffers the output of the AD598, which ensures that the AD7992 ADC is driven by a low impedance source (high source impedances significantly affect the ac performance of the ADC).

The low-pass filter between the output of the AD598 and the input of the AD8615 serves two purposes:

• It limits the input current to the AD8615
• It filters the output voltage ripple.

The AD8615 has internal protective circuitry that allows voltages exceeding the supply to be applied at the input. This is important because the output voltage of the AD598 can swing ±11 V with ±15 V supplies. As long as the input current is limited to less than 5 mA, higher voltages can be applied to the input. This is primarily due to the extremely low input bias current of the AD8615 (1 pA) which allows the use of larger resistors. The use of these resistors adds thermal noise, which contributes to the overall output voltage noise of the amplifier.

The AD8615 is an ideal amplifier to buffer and drive the input of the AD7992 12-bit SAR ADC because of its input overvoltage protection, and its ability to swing rail-to-rail at both the input and output.

Noise Analysis

With all signal condition components selected, the amount of resolution needed to convert the signal must be determined. As in most noise analyses, only the key contributors need to be identified. Noise sources combine in an rss manner; therefore, any single noise source that is at least three-to-four times larger than any of the others dominates.

In the case of the LVDT signal conditioning circuit, the dominant source of the output noise is the output ripple of the AD598. The other sources of noise (resistor noise, input voltage noise, and output voltage noise of the AD8615) are significantly smaller in comparison.

The output voltage ripple of the AD598 is 0.4 mV rms with a 0.39 μF capacitor value and with a 10 nF shunt capacitor across the feedback resistor shown in Figure 2. Note that these components and related pin connections are not shown in the simplified schematic in Figure 1; however, details can be found in the AD598 data sheet.

The maximum number of rms counts that can be resolved can now be calculated by dividing the full-scale output by the total system rms noise.

Total RMS Counts = 5 V/0.4 mV = 12, 500

The effective resolution is found by taking the base 2 logarithm of the total rms counts.

Effective Resolution = log₂(12,500) = 13.6 Bits

Noise-free code resolution can be obtained by subtracting 2.7 bits from the effective resolution.

Noise-Free Code Resolution = Effective Resolution − 2.7 Bits = 13.6 Bits − 2.7 Bits = 10.9 Bits

The total output dynamic range of the system can be calculated by dividing the full-scale output signal (5 V) by the total output rms noise (0.4 mV rms) and converting it to decibels, yielding approximately 82 dB.

Dynamic Range = 20 log(5 V/0.4 mV) = 82 dB

The AD7992 is a good candidate for this application because it has 12-bit resolution and a sampling rate of 188 kSPS per channel when used with a 3.4 MHz serial clock.

Test Results

Using a Measurement Specialties, Inc. E-100 Economy Series LVDT connected to J3 and using a digital oscilloscope to monitor the output of the AD598 found on J6 on the EVALCN0288-SDPZ evaluation board, the actual output ripple found was 6.6 mV p-p, as is shown in Figure 3.

The low-pass filter (3 kΩ, 0.01 μF) between the AD598 output and the AD8615 input has a −3 dB bandwidth of 5.3 kHz and reduces the ripple to 2 mV p-p.

With the low-pass filter installed between the output stage of the AD598 and the input stage of the AD8615, data was collected from the EVAL-CN0288-SDPZ evaluation board, as shown in Figure 4.

The ripple from the AD598 was attenuated to 2 mV p-p, and the system was able to achieve 11 bits of noise-free code resolution.

A complete design support package for this circuit note can be found at http://www.analog.com/CN0288-DesignSupport.

Applications in Flight Control Surface Position Feedback

Unmanned autonomous vehicles (UAVs), or drones, are playing an ever-increasing part in the national security of the United States. These high technology, complex aerial platforms are controlled by a crew miles away and are multimission capable. They include roles such as aerial reconnaissance, combat weapons platforms, battlefield theater command and control oversight, or unmanned in-flight refueling station.

The complex systems employed on UAVs use a myriad of electronic sensors for precise control and feedback. To control the altitude (pitch, roll, and yaw) of the UAV, actuators are used to exert forces on the flight control surfaces. The precise measurement of the position of these actuators is crucial in maintaining the proper flight of path.

The sensors used to measure actuator position need to meet three essential criteria: high accuracy, high reliability, and light weight. All three of these attributes are found in the LVDTs designed by Measurement Specialties, Inc.

Synchronous Operation of Multiple LVDTs

In many applications, such as multiple gaging measurement, a large number of LVDTs are used in close proximity. If these LVDTs operate at similar carrier frequencies, stray magnetic coupling can cause beat notes to be generated. The resulting beat notes may interfere with the accuracy of measurements made under these conditions. To avoid this situation, all LVDTs operate synchronously.

The EVAL-CN0288-SDPZ evaluation board can be configured to have one master oscillator between two LVDTs by populating Jumper JP1, JP2, and JP4 with a shorting jumper and leaving JP3 unpopulated. Each LVDT primary is driven from its own power amplifier, and, thus, the thermal load is shared between the AD598s.