Universal
LVDT Signal Conditioner
a
AD698
FEATURES
FUNCTIO NAL BLO CK D IAGRAM
Single Chip Solution, Contains Internal Oscillator and
Voltage Reference
No Adjustm ents 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 Perform ance
VOLTAGE
REFERENCE
AMP
OSCILLATOR
AD698
B
A
B
AMP
FILTER
Linearity: 0.05%
A
Output Voltage: ؎11 V
Gain Drift: 20 ppm / ؇C (typ)
Offset Drift: 5 ppm / ؇C (typ)
P RO D UCT D ESCRIP TIO N
P RO D UCT H IGH LIGH TS
T he AD698 is a complete, monolithic Linear Variable Differen-
tial T ransformer (LVDT ) signal conditioning subsystem. It is
used in conjunction with LVDT s 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. T he device will operate
with half-bridge LVDT s, LVDT s connected in the series op-
posed configuration (4-wire), and RVDT s.
1. T he 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. T he AD698 can be used with many different types of posi-
tion sensors. T he circuit is optimized for use with any
LVDT , including half-bridge and series opposed, (4 wire)
configurations. T he AD698 accommodates a wide range of
input and output voltages and frequencies.
3. T he 20 Hz to 20 kHz excitation frequency is determined by a
single external capacitor. T he 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.
T he AD698 contains a low distortion sine wave oscillator to
drive the LVDT primary. T wo synchronous demodulation
channels of the AD698 are used to detect primary and second-
ary amplitude. T he part divides the output of the secondary by
the amplitude of the primary and multiplies by a scale factor.
T his eliminates scale factor errors due to drift in the amplitude
of the primary drive, improving temperature performance and
stability.
4. Changes in oscillator amplitude with temperature will not af-
fect overall circuit performance. T he AD698 computes the
ratio of the secondary voltage to the primary voltage to deter-
mine position and direction. No adjustments are required.
T he AD698 uses a unique ratiometric architecture to eliminate
several of the disadvantages associated with traditional ap-
proaches to LVDT interfacing. T he benefits of this new cir-
cuit are: no adjustments are necessary; temperature stability is
improved; and transducer interchangeability is improved.
5. Multiple LVDT s can be driven by a single AD698 either in
series or parallel as long as power dissipation limits are not
exceeded. T he excitation output is thermally protected.
6. T he AD698 may be used as a loop integrator in the design of
simple electromechanical servo loops.
T he AD698 is available in two performance grades:
7. T he sum of the transducer secondary voltages do not need to
be constant.
Gr ade
AD698AP
AD698SQ
Tem per atur e Range
–40°C to +85°C
–55°C to +125°C
P ackage
28-Pin PLCC
24-Pin Cerdip
REV. B
Inform ation furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assum ed by Analog Devices for its
use, nor for any infringem ents of patents or other rights of third parties
which m ay result from its use. No license is granted by im plication or
otherwise under any patent or patent rights of Analog Devices.
© Analog Devices, Inc., 1995
One Technology Way, P.O. Box 9106, Norw ood. MA 02062-9106, U.S.A.
Tel: 617/ 329-4700
Fax: 617/ 326-8703
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