PRODUCT DESCRIPTION

The AD598 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

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 AD598 converts the

raw LVDT secondary output to a scaled dc signal. The device

can also be used with RVDT transducers.

The AD598 contains a low distortion sine wave oscillator to

drive the LVDT primary. 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, producing a scaled unipolar or bipolar dc output.

The AD598 uses a unique ratiometric architecture (patent pend-

ing) to eliminate several of the disadvantages associated with

traditional approaches to LVDT interfacing. The benefits of this

new circuit are: no adjustments are necessary, transformer null

voltage and primary to secondary phase shift does not affect sys-

tem accuracy, temperature stability is improved, and transducer

interchangeability is improved.

The AD598 is available in two performance grades:

Grade

Temperature Range Package

AD598JR

0

°C to +70°C

20-Pin Small Outline (SOIC)

AD598AD –40

°C to +85°C

20-Pin Ceramic DIP

It is also available processed to MIL-STD-883B, for the military

range of –55

°C to +125°C.

FUNCTIONAL BLOCK DIAGRAM

OSC

AMP

AMP

LVDT

EXCITATION (CARRIER)

11

17

10

16

2

3

FILTER

A–B

A+B

AD598

REV. A

Information furnished by Analog Devices is believed to be accurate and

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

LVDT Signal

Conditioner

AD598

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

Tel: 617/329-4700

Fax: 617/326-8703

PRODUCT HIGHLIGHTS

1. The AD598 offers a monolithic solution to LVDT and

RVDT signal conditioning problems; few extra passive com-

ponents are required to complete the conversion from me-

chanical position to dc voltage and no adjustments are

required.

2. The AD598 can be used with many different types of

LVDTs because the circuit accommodates a wide range of

input and output voltages and frequencies; the AD598 can

drive an LVDT primary with up to 24 V rms and accept sec-

ondary input levels as low as 100 mV rms.

3. The 20 Hz to 20 kHz LVDT excitation frequency is deter-

mined by a single external capacitor. The AD598 input sig-

nal need not be synchronous with the LVDT primary drive.

This means that an external primary excitation, such as the

400 Hz power mains in aircraft, can be used.

4. The AD598 uses a ratiometric decoding scheme such that

primary to secondary phase shifts and transducer null voltage

have absolutely no effect on overall circuit performance.

5. Multiple LVDTs can be driven by a single AD598, either in

series or parallel as long as power dissipation limits are not

exceeded. The excitation output is thermally protected.

6. The AD598 may be used in telemetry applications or in hos-

tile environments where the interface electronics may be re-

mote from the LVDT. The AD598 can drive an LVDT at

the end of 300 feet of cable, since the circuit is not affected

by phase shifts or absolute signal magnitudes. The position

output can drive as much as 1000 feet of cable.

7. The AD598 may be used as a loop integrator in the design of

simple electromechanical servo loops.

FEATURES

Single Chip Solution, Contains Internal Oscillator and

Voltage Reference

No Adjustments Required

Insensitive to Transducer Null Voltage

Insensitive to Primary to Secondary Phase Shifts

DC Output Proportional to Position

20 Hz to 20 kHz Frequency Range

Single or Dual Supply Operation

Unipolar or Bipolar Output

Will Operate a Remote LVDT at Up to 300 Feet

Position Output Can Drive Up to 1000 Feet of Cable

Will Also Interface to an RVDT

Outstanding Performance

Linearity: 0.05% of FS max

Output Voltage:

11 V min

Gain Drift: 50 ppm/ C of FS max

Offset Drift: 50 ppm/ C of FS max