AD698APZ Datasheet, PDF (6/12 Pages) Analog Devices – Universal LVDT Signal Conditioner

AD698
CONNECTING THE AD698
The AD698 can easily be connected for dual or single supply
operation as shown in Figures 7, 8 and 13. The 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. The 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.
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
6.8µF
100nF
100nF
R1
C1
15nF
C2
1 –V_{S AD698} +V_S 24
2 EXC1 OFFSET1 23
3 EXC2
OFFSET2 22
4 LEV1
SIG REF 21
5 LEV2
SIG OUT 20
6 FREQ1 FEEDBACK 19
7 FREQ2 OUT FILT 18
R4
R3
SIGNAL
REFERENCE
R2
33kΩ
R_L
V_OUT
C4 1000pF
8 BFILT1
AFILT1 17
C3
9 BFILT2
AFILT2 16
10 –BIN
–ACOMP 15
11 +BIN
+ACOMP 14
12 –AIN
+AIN 13
Figure 7. Interconnection Diagram for Half-Bridge LVDT
and Dual Supply Operation
DESIGN PROCEDURE
DUAL SUPPLY OPERATION
Figure 7 shows the connection method for half-bridge LVDTs.
Figure 8 demonstrates the connections for 3- and 4-wire
LVDTs connected in the series opposed configuration. Both ex-
amples use dual ± 15 volt power supplies.
A. Determine the Oscillator Frequency
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.
1. Determine the mechanical bandwidth required for LVDT
position measurement subsystem, f_SUBSYSTEM. For this ex-
ample, 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. Select excitation frequency determining component C1.
C1 = 35 µF Hz/f_EXCITATION
+15V
–15V
6.8µF
6.8µF
100nF
100nF
R1
C1
C2
1M
1 –V_{S AD698} +V_S 24
2 EXC1 OFFSET1 23
3 EXC2 OFFSET2 22
4 LEV1
SIG REF 21
5 LEV2
SIG OUT 20
6 FREQ1 FEEDBACK 19
7 FREQ2 OUT FILT 18
R4
R3
R2
C4
SIGNAL
REFERENCE
R_L
V_OUT
1000pF
8 BFILT1
9 BFILT2
AFILT1 17
C3
AFILT2 16
10 –BIN
–ACOMP 15
11 +BIN
12 –AIN
+ACOMP 14
+AIN 13
A
B
PHASE
LAG/LEAD
NETWORK
C
D
PHASE LAG
A
B
C
R_S
R_T
R_S
C
D
PHASE LEAD
A
B
R_T
R_S
C
C
C
D
PHASE LAG = Arc Tan (Hz RC);
PHASE LEAD = Arc Tan 1/(Hz RC)
WHERE R = R_S// (R_S + R_T)
Figure 8. AD698 Interconnection Diagram for Series
Opposed LVDT and Dual Supply Operation
B. Determine the Oscillator Amplitude
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. This optimizes linearity and minimizes noise suscepti-
bility. Since the part is ratiometric, the exact value of the excita-
tion is relatively unimportant.
5. Determine optimum LVDT excitation voltage, V_EXC. For a
4-wire LVDT determine the voltage transformation ratio,
VTR, of the LVDT at its mechanical full scale. VTR =
LVDT sensitivity × Maximum Stroke Length from null.
LVDT sensitivity is listed in the LVDT manufacturer’s cata-
log and has units of volts output per volts input per inch dis-
placement. The 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.
–6–
REV. B