PRODUCT DESCRIPTION
The AD698 is a complete, monolithic Linear Variable Differential Transformer (LVDT) signal conditioning subsystem. It is used in conjunction with LVDTs to convert transducer mechanical position to a unipolar or bipolar 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 passive components to set frequency and gain, the AD698 converts the raw LVDT output to a scaled dc signal. The device will operate with half-bridge LVDTs, LVDTs connected in the series opposed configuration (4-wire), and RVDTs.
The AD698 contains a low distortion sine wave oscillator to drive the LVDT primary. Two synchronous demodulation channels of the AD698 are used to detect primary and secondary amplitude. The part divides the output of the secondary by the amplitude of the primary and multiplies by a scale factor. This eliminates scale factor errors due to drift in the amplitude of the primary drive, improving temperature performance and stability.
The AD698 uses a unique ratiometric architecture to eliminate several of the disadvantages associated with traditional approaches to LVDT interfacing. The benefits of this new circuit are: no adjustments are necessary; temperature stability is improved; and transducer interchangeability is improved. The AD698 is available in two performance grades:
Grade Temperature Range Package
AD698AP –40°C to +85°C 28-Pin PLCC
AD698SQ –55°C to +125°C 24-Pin Cerdip
PRODUCT HIGHLIGHTS
1. The AD698 offers a single chip solution to LVDT signal conditioning problems. All active circuits are on the monolithic chip with only passive components required to complete the conversion from mechanical position to dc voltage.
2. The AD698 can be used with many different types of position sensors. The circuit is optimized for use with any LVDT, including half-bridge and series opposed, (4 wire) configurations. The AD698 accommodates a wide range of input and output voltages and frequencies.
3. The 20 Hz to 20 kHz excitation frequency is determined by a single external capacitor. The AD698 provides up to 24 volts rms to differentially drive the LVDT primary, and the AD698 meets its specifications with input levels as low as 100 millivolts rms.
4. Changes in oscillator amplitude with temperature will not affect overall circuit performance. The AD698 computes the ratio of the secondary voltage to the primary voltage to determine position and direction. No adjustments are required.
5. Multiple LVDTs can be driven by a single AD698 either in series or parallel as long as power dissipation limits are not exceeded. The excitation output is thermally protected.
6. The AD698 may be used as a loop integrator in the design of simple electromechanical servo loops.
7. The sum of the transducer secondary voltages do not need to be constant.
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