AD598 Analog Devices, AD598 Datasheet - Page 4

• Manufacturer Part Number: AD598
• Description: LVDT Signal Conditioner
• Manufacturer: Analog Devices

AD598–Typical Characteristics
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. The transducer consists of a primary winding energized by
Figure 3. Gain and Offset CMRR vs. Temperature
Figure 1. Gain and Offset PSRR vs. Temperature
–120
–160
–200
–240
LVDT
Figure 5. AD598 Functional Block Diagram
–10
–15
–20
–25
–30
–35
–40
–80
–5
40
5
0
0
–60
–60
V
V
EXCITATION (CARRIER)
A
B
–20
17
10
–20
11
OFFSET PSRR 12–15V
GAIN PSRR 12–15V
0
0
OFFSET PSRR 15–18V
OFFSET CMRR
TEMPERATURE – C
A–B
A+B
TEMPERATURE – C
OSC
20
20
GAIN CMRR
FILTER
GAIN PSRR 15–18V
AMP
60
60
3V
3
3V
AD598
AMP
100
2
100
(at +25 C and V
16
140
140
V
OUT
–4–
S
=
an external sine wave reference source, two secondary windings
connected in series, and the moveable core to couple flux be-
tween the primary and secondary windings.
The AD598 energizes the LVDT primary, senses the LVDT
secondary output voltages and produces a dc output voltage
proportional to core position. The AD598 consists of a sine
wave oscillator and power amplifier to drive the primary, a de-
coder which determines the ratio of the difference between the
LVDT secondary voltages divided by their sum, a filter and an
output amplifier.
The oscillator comprises a multivibrator which produces a
triwave output. The triwave drives a sine shaper, which pro-
duces a low distortion sine wave whose frequency is determined
by a single capacitor. Output frequency can range from 20 Hz to
20 kHz and amplitude from 2 V rms to 24 V rms. Total har-
monic distortion is typically –50 dB.
The output from the LVDT secondaries consists of a pair of
sine waves whose amplitude difference, (V
to core position. Previous LVDT conditioners synchronously
detect this amplitude difference and convert its absolute value to
15 V, unless otherwise noted)
Figure 4. Typical Offset Drift vs. Temperature
Figure 2. Typical Gain Drift vs. Temperature
120
–20
–40
–60
–80
–10
–20
80
40
20
20
10
0
0
–60
–60
–20
–20
0
0
TEMPERATURE – C
TEMPERATURE – C
20
20
60
60
A
–V
B
100
100
), is proportional
140
140
REV. A