Datasheet
AD598
REV. A
–11–
EXC 1
EXC 2
LEV 1
LEV 2
FREQ 1
FREQ 2
B1 FILT
B2 FILT
OFFSET 1
OFFSET 2
SIG REF
SIG OUT
FEEDBACK
OUT FILT
A1 FILT
A2 FILT
AD598
–V
S
+V
S
1
2
3
4
5
6
7
8
9
10 11
12
13
14
16
15
17
18
19
20
V
B
V
A
–
V
+
V
0.015µF
0.1µF
15k 15k
82.5kΩ
0.33µF
0.1µF
SCHAEVITZ E 100
MECHANICAL POSITION INPUT
EXC 1
EXC 2
LEV 1
LEV 2
FREQ 1
FREQ 2
B1 FILT
B2 FILT
OFFSET 1
OFFSET 2
SIG REF
SIG OUT
FEEDBACK
OUT FILT
A1 FILT
A2 FILT
AD598
–V
S
+V
S
1
2
3
4
5
6
7
8
9
10 11
12
13
14
16
15
17
18
19
20
V
B
V
A
–
V
+
V
SCHAEVITZ E 100
MECHANICAL POSITION INPUT
0.1µF
82.5kΩ
0.33µF
0.1µF
15k 15k
EXC 1
EXC 2
LEV 1
LEV 2
FREQ 1
FREQ 2
B1 FILT
B2 FILT
OFFSET 1
OFFSET 2
SIG REF
SIG OUT
FEEDBACK
OUT FILT
A1 FILT
A2 FILT
AD598
–V
S
+V
S
1
2
3
4
5
6
7
8
9
10 11
12
13
14
16
15
17
18
19
20
V
B
V
A
–
V
+
V
SCHAEVITZ E 100
MECHANICAL POSITION INPUT
0.1µF
82.5kΩ
0.33µF
0.1µF
MASTER SLAVE 1 SLAVE 2
LVDT LVDT LVDT
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. The V/F
converter offset voltage is normally the largest source of error in
such circuits. The analog input signal to the AD652 is converted
to digital frequency output pulses which can be counted by
simple digital means.
This circuit is particularly useful if there is a large degree of
mechanical vibration (hum) on the position to be measured.
The hum may be completely rejected by counting the digital fre-
quency pulses over a gate time (fixed period) equal to a multiple
of the hum period. For the effects of the hum to be completely
rejected, the hum must be a periodic signal.
EXC 1
EXC 2
LEV 1
LEV 2
FREQ 1
FREQ 2
B1 FILT
B2 FILT
OFFSET 1
OFFSET 2
SIG REF
SIG OUT
FEEDBACK
OUT FILT
A1 FILT
A2 FILT
AD598
–V
S
+V
S
1
2
3
4
5
6
7
8
9
10 11
12
13
14
16
15
17
18
19
20
V
B
V
A
0.015µF
0.1µF
0.33µF
0.1µF
SCHAEVITZ E 100
MECHANICAL POSITION INPUT
–
Vs
0.1µF
GND
0.1µF
+
Vs
1
2
3
4
5
6
7
8
15
14
13
12
11
9
10
16
+V
S
2.5k
+V
S
FREQ
OUT
500KHZ
CK
0.02µF
AD652
SYNCHRONOUS
VOLTAGE TO
FREQUENCY
CONVERTER
+V
S
TRIM
TRIM
OP AMP OUT
OP AMP “–”
OP AMP “+”
10 VOLT INPUT
–V
S
C
OS
CLOCK INPUT
FREQ OUT
DIGITAL GND
ANALOG GND
COMP“–”
COMP“+”
COMP REF
LVDT
Figure 22. High Resolution Position-to-Frequency Converter