REV. B

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reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

a

Universal

LVDT Signal Conditioner

AD698

© Analog Devices, Inc., 1995

One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A.

Tel: 617/329-4700

Fax: 617/326-8703

FEATURES

Single Chip Solution, Contains Internal Oscillator and

Voltage Reference

No Adjustments Required

Interfaces to Half-Bridge, 4-Wire LVDT

DC Output Proportional to Position

20 Hz to 20 kHz Frequency Range

Unipolar or Bipolar Output

Will Also Decode AC Bridge Signals

Outstanding Performance

Linearity: 0.05%

Output Voltage:

11 V

Gain Drift: 20 ppm/ C (typ)

Offset Drift: 5 ppm/ C (typ)

PRODUCT DESCRIPTION

The AD698 is a complete, monolithic Linear Variable Differen-

tial Transformer (LVDT) signal conditioning subsystem. It is

used in conjunction with LVDTs to convert transducer mechan-

ical position to a unipolar or bipolar dc voltage with a high de-

gree 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 op-

posed 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 second-

ary 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 ap-

proaches to LVDT interfacing. The benefits of this new cir-

cuit 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 mono-

lithic chip with only passive components required to com-

plete the conversion from mechanical position to dc voltage.

2. The AD698 can be used with many different types of posi-

tion 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 af-

fect overall circuit performance. The AD698 computes the

ratio of the secondary voltage to the primary voltage to deter-

mine 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.

FUNCTIONAL BLOCK DIAGRAM

A

B

AMP

OSCILLATOR

VOLTAGE

REFERENCE

A

B

FILTER

AMP

AD698