AD698
CO NNECTING TH E AD 698
3. Select a suitable LVDT that will operate with an excitation
frequency of 2.5 kHz. T he Schaevitz E100, for instance, will
operate over a range of 50 Hz to 10 kHz and is an eligible
candidate for this example.
T he AD698 can easily be connected for dual or single supply
operation as shown in Figures 7, 8 and 13. T he following gen-
eral design procedures demonstrate how external component
values are selected and can be used for any LVDT that meets
AD698 input/output criteria. T he connections for the A and B
channels and the A channel comparators will depend on which
transducer is used. In general follow the guidelines below.
4. Select excitation frequency determining component C1.
C1 = 35 µF Hz/fEXCITATION
Parameters set with external passive components include: exci-
tation frequency and amplitude, AD698 input signal frequency,
and the scale factor (V/inch). Additionally, there are optional
features; offset null adjustment, filtering, and signal integration,
which can be implemented by adding external components.
+15V
–15V
6.8µF
100nF
100nF
6.8µF
–V
1
2
24
S
+V
S
AD698
R4
R3
23
EXC1
EXC2
LEV1
LEV2
OFFSET1
OFFSET2
SIG REF
SIG OUT
22
21
20
3
4
SIGNAL
REFERENCE
+15V
–15V
6.8µF
100nF
R
L
R1
C1
C2
100nF
6.8µF
5
V
R2
OUT
–V
1
2
24
S
+V
S
6
AD698
FREQ1 FEEDBACK 19
R4
R3
C4
1000pF
23
EXC1
EXC2
LEV1
LEV2
OFFSET1
OFFSET2
SIG REF
SIG OUT
18
OUT FILT
7
FREQ2
BFILT1
BFILT2
–BIN
22
21
20
3
4
SIGNAL
REFERENCE
8
AFILT1 17
AFILT2 16
C3
9
R
R1
L
5
V
R2
33kΩ
10
11
15
–ACOMP
+ACOMP 14
13
OUT
6
FREQ1 FEEDBACK 19
+BIN
C1
1000pF
C4
A
B
15nF
18
OUT FILT
7
FREQ2
BFILT1
BFILT2
–BIN
12 –AIN
+AIN
PHASE
LAG/LEAD
NETWORK
8
AFILT1 17
AFILT2 16
C3
C2
9
1M
10
11
15
–ACOMP
+ACOMP 14
13
D
C
+BIN
PHASE LEAD
PHASE LAG
A
12 –AIN
+AIN
A
B
B
PHASE LAG = Arc Tan (Hz RC);
C
R
R
T
S
PHASE LEAD = Arc Tan 1/(Hz RC)
WHERE R = R // (R + R )
R
S
S
T
T
C
R
C
R
S
S
Figure 7. Interconnection Diagram for Half-Bridge LVDT
and Dual Supply Operation
C
D
C
D
Figure 8. AD698 Interconnection Diagram for Series
Opposed LVDT and Dual Supply Operation
D ESIGN P RO CED URE
D UAL SUP P LY O P ERATIO N
Figure 7 shows the connection method for half-bridge LVDT s.
Figure 8 demonstrates the connections for 3- and 4-wire
LVDT s connected in the series opposed configuration. Both ex-
B. D eter m ine the O scillator Am plitude
Amplitude is set such that the primary signal is in the 1.0 V to
3.5 V rms range and the secondary signal is in the 0.25 V to
3.5 V rms range when the LVDT is at its mechanical full-scale
position. T his optimizes linearity and minimizes noise suscepti-
bility. Since the part is ratiometric, the exact value of the excita-
tion is relatively unimportant.
amples use dual ±15 volt power supplies.
A. D eter m ine the O scillator Fr equency
Frequency is often determined by the required BW of the sys-
tem. However, in some systems the frequency is set to match
the LVDT zero phase frequency as recommended by the
manufacturer; in this case skip to Step 4.
5. Determine optimum LVDT excitation voltage, VEXC. For a
4-wire LVDT determine the voltage transformation ratio,
VT R, of the LVDT at its mechanical full scale. VT R =
LVDT sensitivity × Maximum Stroke Length from null.
1. Determine the mechanical bandwidth required for LVDT
position measurement subsystem, fSUBSYST EM. For this ex-
ample, assume fSUBSYST EM = 250 Hz.
LVDT sensitivity is listed in the LVDT manufacturer’s cata-
log and has units of volts output per volts input per inch dis-
placement. T he E100 has a sensitivity of 2.4 mV/V/mil. In
the event that LVDT sensitivity is not given by the manufac-
turer, it can be computed. See section on determining LVDT
sensitivity.
2. Select minimum LVDT excitation frequency approximately
10 × fSUBSYSTEM. Therefore, let excitation frequency = 2.5 kHz.
–6–
REV. B
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