Datasheet

AD598
REV. A
–14–
PRECISION DIFFERENTIAL GAGING
The circuit shown in Figure 26 is functionally similar to the dif-
ferential gaging circuit shown in Figure 25. In contrast to Figure
25, it provides a means of independently adjusting the scale fac-
tor of each LVDT so that both scale factors may be matched.
The two LVDTs are driven in a master-slave arrangement
where the output signal from the slave LVDT is summed with
the output signal from the master LVDT. The scale factor of the
slave LVDT only is adjusted with R1 and R2. The summed
scale factor of the master LVDT and the slave LVDT is ad-
justed with R3.
R1 and R2 are chosen to be 80.9 kΩ resistors to give a ±10 V
full-scale output signal for a single Schaevitz E100 LVDT. R3 is
chosen to be 40.2 kΩ to give a ±10 V output signal when the
two E100 LVDT output signals are summed. The output volt-
age for this circuit is given by:
V
OUT
=
(V
A
–V
B
)
(V
A
+V
B
)
+
(V
C
–V
D
)
(V
C
+V
D
)
×
R2
R1






× 500 µA × R3.
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
0.1µF
0.33µF
0.1µF
–
V
0.1µF 0.1µF
+
V
A
B
C
D
SCHAEVITZ E 100
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
–
V
0.1µF 0.1µF
+
V
R3 40.2kΩ
SLAVE
LVDT
MASTER LVDT
15kΩ
15kΩ
R2
80.9kΩ
R1
80.9kΩ
V
OUT
=
V A –V B
V A +V B
+
V C –V D
V C +V D
••
R2
R1
500µA
•
R3
V
OUT
±
10V
FULL SCALE
Figure 26. Precision Differential Gaging