AD2S93BP AD [Analog Devices], AD2S93BP Datasheet - Page 8

• Manufacturer Part Number: AD2S93BP
• Description: Low Cost LVDT-to-Digital Converter
• Manufacturer: AD [Analog Devices]

AD2S93
Three- or Four-Wire LVDT Connection
In this method of connection, shown in Figure 6, the converters
digital output is proportional to the ratio:
where A and B are the individual LVDT secondary output volt-
ages. Inspection of Figure 6 should demonstrate why this rela-
tionship is true. (A–B) is simply the voltage across the series
connected secondaries of the LVDT and is applied to the A, B
input to the converter. (A + B)/2 is effectively the average of
the two secondary voltages as computed by the balanced bridge
completion resistors and the grounding of the secondary
center-tap.
Note: This method of connection is appropriate only for where
(A + B) is a constant, independent of LVDT position. Any lack
of constancy in (A + B) will be reflected as an additional non-
PISTON
PISTON
( A
( A
B ) / 2
B )
Figure 6. Three- or Four-Wire LVDT Connection
R
R
A
B
Figure 5. Half-Bridge Type LVDT Connection
C1
C2
R
R
C1
C2
GND
R4
V
DD
R4
V
12k
DD
R3
12k
AGND
R3
GAIN
DIFF
LOS
AGND
NC
GAIN
DIFF
LOS
B
A
NC
B
A
REF
26
27
28
1
2
3
4
–8–
REF
26
28
27
1
2
3
4
linearity in the output. It is up to the user to determine if (A +
B) is sufficiently constant over the particular stroke length em-
ployed.
This method will usually restrict the usable LVDT range to half
of its full range. The restriction can be eliminated, however, by
attenuating DIFF by a factor of 2 or increasing V
of 2. This connection method has the tremendous advantage of
being insensitive to temperature related phase shifts and excita-
tion oscillator instability effects usually associated with more
conventional LVDT conversion systems.
As in the case of the half-bridge type LVDT connection, R
and R
a degree sufficient to ensure that the digital output representing
the null position does not vary from the LVDT’s natural null
position. If null adjustment is required, a potentiometer can be
used in place of the common connection between the two
resistors.
25 24
5
25 24
5
6
C2
6
C1
(Not to Scale)
23 22
7
R2
AD2S93
TOP VIEW
are the bridge completion resistors and are matched to
C1
23
(Not to Scale)
C2
R2
7
TOP VIEW
AD2S93
8
C2
22
8
21
9
21
9
10
20
20
10
R6
11
19
R6
C3
19
11
C3
18
17
16
15
14
13
12
NC = NO CONNECT
NC = NO CONNECT
18
17
16
15
14
13
12
C4
DIR
R5
DGND
NULL
OVR
V
V
DD
SS
DEMODOUT
R7
DGND
DIR
NULL
OVR
C4
V
R5
V
R7
DEMODOUT
DD
SS
+5V
–5V
0V
+5V
–5V
0V
REF
by a factor
REV. A
C1