Datasheet

AD598
REV. A
–15–
OPERATION WITH A HALF-BRIDGE TRANSDUCER
Although the AD598 is not intended for use with a half-bridge
type transducer, it may be made to function with degraded
performance.
A half-bridge type transducer is a popular transducer. It works
in a similar manner to the LVDT in that two coils are wound
around a moveable core and the inductance of each coil is a
function of core position.
In the circuit shown in Figure 27 the V
A
and V
B
input voltages
are developed as two resistive-inductor dividers. If the inductors
are equal (i.e., the core is centered), the V
A
and V
B
input volt-
ages to the AD598 are equal and the output voltage V
OUT
is
zero. When the core is positioned off center, the inductors are
unequal and an output voltage V
OUT
is developed.
The linearity of this circuit is dependent upon the value of the
resistors in the resistive-inductor dividers. The optimum value
may be transducer dependent and therefore must be selected by
trial and error. The 300 Ω resistors in this circuit optimize the
nonlinearity of the transfer function to within several tenths of
1%. This circuit uses a Sangamo AGH1 half-bridge transducer.
The 1 µF capacitor blocks the dc offset of the excitation output
signal. The 4 nF capacitor sets the transducer excitation fre-
quency to 10 kHz as recommended by the manufacturer.
ALTERNATE HALF-BRIDGE TRANSDUCER CIRCUIT
This circuit suffers from similar accuracy problems to those
mentioned in the previous circuit description. In this circuit the
V
A
input signal to the AD598 really and truly is a linear function
of core position, and the input signal V
B
, is one half of the exci-
tation voltage level. However, a nonlinearity is introduced by
the A–B/A+B transfer function.
The 500 Ω resistors in this circuit are chosen to minimize errors
caused by dc bias currents from the V
A
and V
B
inputs. Note that
in the previous circuit these bias currents see very low resistance
paths to ground through the coils.
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
82.5kΩ
5kΩ
4nF
SANGAMO
AGHI
HALF-BRIDGE
1µF1µF
300Ω 300Ω
MECHANICAL
POSITION
INPUT
V
OUT
±
10V
FULL SCALE
Figure 27. Half-Bridge Operation
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
143kΩ
1.87kΩ
4nF
SANGAMO
AGHI
HALF-BRIDGE
1µF
500Ω
500Ω
MECHANICAL
POSITION
INPUT
V
OUT
±
10V
FULL SCALE
Figure 28. Alternate Half-Bridge Circuit