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:
(A − B)
(A + B) / 2
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-
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 VREF by a factor
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, RC1
and RC2 are the bridge completion resistors and are matched to
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.
PISTON
RC1
RC2
A
B
C1
C2
C4
C3
R2
R6
R5
REF
25 24 23 22 21 20 19
R7
NC 26
B 27
GND
28
AGND 1
DIFF 2
R4 R3
GAIN 3
12kΩ
LOS 4
VDD
AD2S93
TOP VIEW
(Not to Scale)
18 DEMODOUT
17 VDD
+5V
0V
16 VSS
–5V
15 DGND
14 DIR
13 NULL
12 OVR
5 6 7 8 9 10 11
NC = NO CONNECT
PISTON
Figure 5. Half-Bridge Type LVDT Connection
C1
C2
C3
R2
R6
REF
25 24 23 22 21 20 19
C4
R5
R7
NC 26
B 27
28
RC1
AGND 1
DIFF 2
RC2
R4 R3
GAIN 3
12kΩ
LOS 4
VDD
AD2S93
TOP VIEW
(Not to Scale)
18 DEMODOUT
17 VDD
+5V
0V
16 VSS
–5V
15 DGND
14 DIR
13 NULL
12 OVR
5 6 7 8 9 10 11
NC = NO CONNECT
Figure 6. Three- or Four-Wire LVDT Connection
–8–
REV. A