AD598
DESIGN PROCEDURE
DUAL SUPPLY OPERATION
Figure 7 shows the connection method with dual
±15
volt power
supplies and a Schaevitz E100 LVDT. This design procedure
can be used to select component values for other LVDTs as
well. The procedure is outlined in Steps 1 through 10 as follows:
1. Determine the mechanical bandwidth required for LVDT
position measurement subsystem, f
SUBSYSTEM
. For this
example, assume f
SUBSYSTEM
= 250 Hz.
2. Select minimum LVDT excitation frequency, approximately
10
×
f
SUBSYSTEM
. Therefore, let excitation frequency = 2.5 kHz.
3. Select a suitable LVDT that will operate with an excitation
frequency of 2.5 kHz. The Schaevitz E100, for instance, will
operate over a range of 50 Hz to 10 kHz and is an eligible
candidate for this example.
4. Determine the sum of LVDT secondary voltages V
A
and V
B
.
Energize the LVDT at its typical drive level V
PRI
as shown in
the manufacturer’s data sheet (3 V rms for the E100). Set the
core displacement to its center position where V
A
= V
B
. Mea-
sure these values and compute their sum V
A
+V
B
. For the
E100, V
A
+V
B
= 2.70 V rms. This calculation will be used
later in determining AD598 output voltage.
5. Determine optimum LVDT excitation voltage, V
EXC
. With
the LVDT energized at its typical drive level V
PRI
, set the
core displacement to its mechanical full-scale position and
measure the output V
SEC
of whichever secondary produces
the largest signal. Compute LVDT voltage transformation
ratio, VTR.
VTR = V
PRI
/V
SEC
For the E100, V
SEC
= 1.71 V rms for V
PRI
= 3 V rms.
VTR = 1.75.
The AD598 signal input, V
SEC
, should be in the range of
1 V rms to 3.5 V rms for maximum AD598 linearity and
minimum noise susceptibility. Select V
SEC
= 3 V rms. There-
fore, LVDT excitation voltage V
EXC
should be:
V
EXC
=
V
SEC
×
VTR
= 3
×
1.75 = 5.25
V rms
Check the power supply voltages by verifying that the peak
values of V
A
and V
B
are at least 2.5 volts less than the volt-
ages at +V
S
and –V
S
.
6. Referring to Figure 7, for V
S
=
±
15 V, select the value of the
amplitude determining component R1 as shown by the curve
in Figure 8.
7. Select excitation frequency determining component C1.
C1 = 35
µ
F Hz/f
EXCITATION
30
20
V
EXC
– V
rms
V
rms
10
0
0.01
0.1
1
R1 – kΩ
10
100
1000
Figure 8. Excitation Voltage V
EXC
vs. R1
+15V
6.8µF
0.1µF
6.8µF
0.1µF
1 –V
S
2 EXC 1
3 EXC 2
4 LEV 1
R1
5 LEV 2
6 FREQ 1
C1
7 FREQ 2
8 B1 FILT
C2
9 B2 FILT
V
B
10 V
B
A2 FILT 12
V 11
A
NOTE
FOR C1, C2, C3 AND C4 MYLAR
CAPACITORS ARE
RECOMMENDED. CERAMIC
CAPACITORS MAY BE
SUBSTITUTED. FOR R2, R3 AND
R4 USE STANDARD 1%
RESISTORS.
OUT FILT 14
A1 FILT 13
C3
SIG OUT 16
R2
FEEDBACK 15
C4
V
OUT
+V
S
20
R4
OFFSET 1 19
OFFSET 2 18
R3
SIG REF 17
R
L
SIGNAL
REFERENCE
–15V
AD598
V
A
SCHAEVITZ E100
LVDT
Figure 7. Interconnection Diagram for Dual Supply Operation
–6–
REV. A
AD [ ANALOG DEVICES ]
