–8–

REV. A

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

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.

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.

NC = NO CONNECT

NC

OVR

NULL

DEMODOUT

DIR

26

27

28

4

2

3

15

18

17

16

12

14

13

25

24

21

20

19

23

22

1

5

69

10

11

7

8

TOP VIEW

(Not to Scale)

AD2S93

+5V

0V

–5V

LOS

12k

Ω

GAIN

DIFF

AGND

A

B

R4

R3

V

DD

R

C2

R

C1

B

A

R2

C2

C1

C3

R5

REF

PISTON

GND

V

DD

V

SS

DGND

R6

R7

C4

Figure 5. Half-Bridge Type LVDT Connection

NC = NO CONNECT

NC

OVR

NULL

DEMODOUT

DIR

26

27

28

4

2

3

15

18

17

16

12

14

13

25

24

21

20

19

23

22

1

5

6

9

10

11

7

8

TOP VIEW

(Not to Scale)

AD2S93

V

DD

DGND

V

SS

+5V

0V

–5V

C3

R5

LOS

12k

Ω

GAIN

DIFF

AGND

A

B

R4

R3

PISTON

V

DD

REF

C4

R6

R2

C2

C1

R7

R

C2

R

C1

Figure 6. Three- or Four-Wire LVDT Connection