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