AD598 Analog Devices, AD598 Datasheet

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AD598

Manufacturer Part Number
AD598
Description
LVDT Signal Conditioner
Manufacturer
Analog Devices
Datasheet

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a
FEATURES
Single Chip Solution, Contains Internal Oscillator and
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
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
AD598JR 0 C to +70 C
AD598AD –40 C to +85 C
It is also available processed to MIL-STD-883B, for the military
range of –55 C to +125 C.
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.
Voltage Reference
Linearity: 0.05% of FS max
Output Voltage:
Gain Drift: 50 ppm/ C of FS max
Offset Drift: 50 ppm/ C of FS max
Temperature Range Package
11 V min
20-Pin Small Outline (SOIC)
20-Pin Ceramic DIP
PRODUCT HIGHLIGHTS
1. The AD598 offers a monolithic solution to LVDT and
2. The AD598 can be used with many different types of
3. The 20 Hz to 20 kHz LVDT excitation frequency is deter-
4. The AD598 uses a ratiometric decoding scheme such that
5. Multiple LVDTs can be driven by a single AD598, either in
6. The AD598 may be used in telemetry applications or in hos-
7. The AD598 may be used as a loop integrator in the design of
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
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.
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.
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.
primary to secondary phase shifts and transducer null voltage
have absolutely no effect on overall circuit performance.
series or parallel as long as power dissipation limits are not
exceeded. The excitation output is thermally protected.
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.
simple electromechanical servo loops.
LVDT
FUNCTIONAL BLOCK DIAGRAM
V
V
EXCITATION (CARRIER)
A
B
17
10
11
A–B
A+B
OSC
FILTER
AMP
Conditioner
LVDT Signal
3
AD598
AMP
2
AD598
Fax: 617/326-8703
16
V
OUT

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AD598 Summary of contents

Page 1

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

Page 2

... V rms 4.1 2.6 4.1 V rms rms 600 ppm rms 12 mA rms 60 mA 100 30 100 mV 20k 20 20k Hz 200 ppm/ C –50 dB 3.5 0.1 3.5 V rms 200 kHz 17 +70 C –40 +85 C AD598AD REV. A ...

Page 3

... S S Storage Temperature Range R Package . . . . . . . . . . . . . . . . . . . . . . . . . – +150 C D Package . . . . . . . . . . . . . . . . . . . . . . . . . – +150 C Operating Temperature Range AD598JR . . . . . . . . . . . . . . . . . . . . . . . . . . . . +70 C AD598AD . . . . . . . . . . . . . . . . . . . . . . . . . . – +85 C Lead Temperature Range (Soldering 60 sec +300 C Power Dissipation 1.2 W Derates Above + mW/ C REV 1.65% of full scale. Note that 1000 ppm of full scale equals 0.1% of full scale. ...

Page 4

... TEMPERATURE – C Figure 3. Gain and Offset CMRR vs. Temperature THEORY OF OPERATION A block diagram of the AD598 along with an LVDT (Linear Variable Differential Transformer) connected to its input is shown in Figure 5. The LVDT is an electromechanical trans- ducer whose input is the mechanical displacement of a core and whose output is a pair of ac voltages proportional to core posi- tion ...

Page 5

... The AD598 can easily be connected for dual or single supply operation as shown in Figures 7 and 12. The following general design procedures demonstrate how external component values are selected and can be used for any LVDT which meets AD598 input/output criteria. Parameters which are set with external passive components in- clude: excitation frequency and amplitude, AD598 system bandwidth, and the scale factor (V/inch) ...

Page 6

... SCHAEVITZ E100 LVDT Figure 7. Interconnection Diagram for Dual Supply Operation The AD598 signal input rms to 3.5 V rms for maximum AD598 linearity and minimum noise susceptibility. Select V fore, LVDT excitation voltage V V Check the power supply voltages by verifying that the peak values of V ...

Page 7

... Farad Hz/250 Hz = 0.4 F See Figures 13, 14 and 15 for more information about AD598 bandwidth and phase characterization order to Compute R2, which sets the AD598 gain or full- scale output range, several pieces of information are needed: a. LVDT sensitivity Full-scale core displacement ...

Page 8

... The figures are transfer functions with the input to be considered as a sinusoidally varying mechanical posi- tion and the output as the voltage from the AD598; the units of the transfer function are volts per inch. The value of C2, C3 and C4, from Figure 7, are all equal and designated as a parameter in the figures ...

Page 9

... Figure 18. LVDT Secondary Voltage vs. Core Displacement 1 10 Thermal Shutdown and Loading Considerations The AD598 is protected by a thermal overload circuit. If the die temperature reaches 165 C, the sine wave excitation amplitude gradually reduces, thereby lowering the internal power dissipa- tion and temperature. Due to the ratiometric operation of the decoder circuit, only small errors result from the reduction of the excitation ampli- tude ...

Page 10

... The value can be calculated using one of two sepa- rate methods. First, a potentiometer may be connected between Pins 18 and 19 of the AD598, with the wiper connected to –V . This gives a small offset of either polarity; and the SUPPLY value can be calculated using Step 10 of the design procedures ...

Page 11

... MECHANICAL POSITION INPUT Figure 21. Multiple LVDTs—Synchronous Operation HIGH RESOLUTION POSITION-TO-FREQUENCY CIRCUIT In the circuit shown in Figure 22, the AD598 is combined with an AD652 voltage-to-frequency (V/F) converter to produce an effective, simple data converter which can make high resolution measurements. This circuit transfers the signal from the LVDT to the V/F con- verter in the form of a current, thus eliminating the errors nor- mally caused by the offset voltage of the V/F converter ...

Page 12

... This circuit is both simple and inexpensive. It has the advantage that two LVDTs may be driven from one AD598, but the disad- vantage is that the scale factor of each LVDT may not match exactly. This causes the workpiece thickness measurement to vary depending upon its absolute position in the differential gage head ...

Page 13

... GND IN4740A 10V POWER SUPPLY + V 0 OFFSET OFFSET 2 17 SIG REF V 10V OUT 16 SIG OUT FULL SCALE R2 80.9k 15 FEEDBACK 0. OUT FILT 13 A1 FILT 0 FILT AD598 )+(V –V ) • 500 A • )+( ...

Page 14

... AD598 80. 0 OFFSET 1 OFFSET SIG REF 80.9k 16 SIG OUT 15 FEEDBACK 0.33 F OUT FILT FILT 0 FILT AD598 –V V – • • • 500 10V OUT REV. A ...

Page 15

... ALTERNATE HALF-BRIDGE TRANSDUCER CIRCUIT This circuit suffers from similar accuracy problems to those and V input voltages mentioned in the previous circuit description. In this circuit the B V input signal to the AD598 really and truly is a linear function A and V input volt- of core position, and the input signal tation voltage level ...

Page 16

... AD598 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 20-Pin Sized Brazed Ceramic DIP 20-Lead Wide Body Plastic SOIC (R) Package –16– REV. A ...

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