–9–

REV. A

AD2S93

Two-Wire LVDT Connection

This method should be used in cases where the sum of the

LVDT secondary output voltages (A + B) is not constant with

LVDT displacement over the desired stroke length. This method

of connection, shown in Figure 7, still maintains the ratiometric

operation and the insensitivity to variations in reference ampli-

and V1 should be minimized to maintain accuracy (see Section

“PHASE SHIFT AND QUADRATURE EFFECTS”). Sug-

gested phase compensation circuits are shown in Figure 7.

PHASE SHIFT AND QUADRATURE EFFECTS

Reference to signal phase shift can be high in LVDTs, some-

times in the order of 70 degrees. If the converter is connected

as in Figures 5 and 6, any effects due to this phase shift are

minimized. This connection method, therefore, provides out-

standing benefits.

The additional gain error caused by reference to signal phase

shifts is given by:

(1 – cos

θ) × 100% of FSR

where

θ = phase shift between V

quadrature on the signal will have no effect on the converter.

This is another benefit of the conversion method. For example,

when a REF lags (A–B) by approximately 10

°, the gain error is

approximately 1%. When (A–B) lags REF by approximately

10

°, the gain error is approximately 2%.

REMOTE MULTIPLE SENSOR INTERFACING

The DATA output of the AD2S93 is held in a high impedance

state until CS is taken LO. This allows a user to operate the

AD2S93 in an application with more than one converter con-

nected on the same line. Figure 8 shows four LVDTs interfaced

to four AD2S93s. Excitation for the LVDT is provided locally

by an oscillator.

SCLK, DATA and two address lines are fed down low loss

cables suitable for communication links. The two address lines

are decoded locally into CS for the individual converters. Data

is received and transmitted using transmitters and receivers.

2-4 DECODING

(74HC139)

LVDT

LVDT

LVDT

LVDT

AD2S93

1

AD2S93

2

AD2S93

3

AD2S93

4

OSC

BUFFER

4

4

4

4

2

2

0V

A0

A1

DATA

SCLK

Figure 8. Remote Sensor Interface

NC = NO CONNECT

OVR

NULL

DEMODOUT

DIR

4

2

3

25

24

21

20

19

23

22

1

5

69

10

7

8

TOP VIEW

(Not to Scale)

AD2S93

+5V

0V

–5V

R4

R3

PHASE

SHIFT

CCT

OSC

PISTON

V

DD

PHASE LAG = ARCTAN 2

π fRC

4

2

3

25

24

21

20

19

23

22

1

5

69

10

7

8

TOP VIEW

(Not to Scale)

AD2S93

12k

Ω

PHASE

SHIFT

CCT

PHASE LEAD = ARCTAN

2

π fRC

1

V

DD

V

SS

DGND

AGND

LOS

GAIN

DIFF

11

NC

26

27

28

A

B

12

15

18

17

16

14

13

REF

C

R

R

C

C3

R5

C4

R6

R2

C2

C1

R7

Figure 7. Two-Wire LVDT Connection