(at +25؇C and V = ؎15 V, unless otherwise noted)
AD598–Typical Characteristics
S
40
120
80
OFFSET PSRR 12–15V
0
OFFSET PSRR 15–18V
–40
40
GAIN PSRR 12–15V
–80
20
–120
–160
–200
–240
0
–20
–40
–60
–80
GAIN PSRR 15–18V
–60
–20
0
20
60
100
140
–60
–20
0
20
60
100
140
TEMPERATURE – °C
TEMPERATURE – °C
Figure 1. Gain and Offset PSRR vs. Temperature
Figure 2. Typical Gain Drift vs. Temperature
5
20
0
OFFSET CMRR ± 3V
–5
–10
–15
–20
10
0
GAIN CMRR ± 3V
–25
–10
–20
–30
–35
–60
–20
0
20
60
100
140
–60
–20
0
20
60
100
140
TEMPERATURE – °C
TEMPERATURE – °C
Figure 3. Gain and Offset CMRR vs. Temperature
Figure 4. Typical Offset Drift vs. Temperature
THEORY OF OPERATION
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.
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
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.
EXCITATION (CARRIER)
3
2
V
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.
A
11
17
10
AMP
OSC
AD598
A–B
A+B
VOUT
16
FILTER
AMP
The output from the LVDT secondaries consists of a pair of
sine waves whose amplitude difference, (VA–VB), is proportional
to core position. Previous LVDT conditioners synchronously
detect this amplitude difference and convert its absolute value to
LVDT
VB
Figure 5. AD598 Functional Block Diagram
–4–
REV. A
ADI [ ADI ]
