AD598

REV. A

–15–

OPERATION WITH A HALF-BRIDGE TRANSDUCER

Although the AD598 is not intended for use with a half-bridge

type transducer, it may be made to function with degraded

performance.

A half-bridge type transducer is a popular transducer. It works

in a similar manner to the LVDT in that two coils are wound

around a moveable core and the inductance of each coil is a

function of core position.

are developed as two resistive-inductor dividers. If the inductors

zero. When the core is positioned off center, the inductors are

The linearity of this circuit is dependent upon the value of the

resistors in the resistive-inductor dividers. The optimum value

may be transducer dependent and therefore must be selected by

trial and error. The 300

Ω resistors in this circuit optimize the

nonlinearity of the transfer function to within several tenths of

1%. This circuit uses a Sangamo AGH1 half-bridge transducer.

The 1

µF capacitor blocks the dc offset of the excitation output

signal. The 4 nF capacitor sets the transducer excitation fre-

quency to 10 kHz as recommended by the manufacturer.

ALTERNATE HALF-BRIDGE TRANSDUCER CIRCUIT

This circuit suffers from similar accuracy problems to those

mentioned in the previous circuit description. In this circuit the

tation voltage level. However, a nonlinearity is introduced by

the A–B/A+B transfer function.

The 500

Ω resistors in this circuit are chosen to minimize errors

in the previous circuit these bias currents see very low resistance

paths to ground through the coils.

EXC 1

EXC 2

LEV 1

LEV 2

FREQ 1

FREQ 2

B1 FILT

B2 FILT

OFFSET 1

OFFSET 2

SIG REF

SIG OUT

FEEDBACK

OUT FILT

A1 FILT

A2 FILT

AD598

1

2

3

4

5

6

7

8

9

10

11

12

13

14

16

15

17

18

19

20

0.1

µF

0.33

µF

0.1

µF

–V

0.1

µF

0.1

µF

+V

82.5k

Ω

5k

Ω

4nF

SANGAMO

AGHI

HALF-BRIDGE

1

µF1µF

300

Ω

300

Ω

MECHANICAL

POSITION

INPUT

FULL SCALE

Figure 27. Half-Bridge Operation

EXC 1

EXC 2

LEV 1

LEV 2

FREQ 1

FREQ 2

B1 FILT

B2 FILT

OFFSET 1

OFFSET 2

SIG REF

SIG OUT

FEEDBACK

OUT FILT

A1 FILT

A2 FILT

AD598

1

2

3

4

5

6

7

8

9

10

11

12

13

14

16

15

17

18

19

20

0.1

µF

0.33

µF

0.1

µF

–V

0.1

µF

0.1

µF

+V

143k

Ω

1.87k

Ω

4nF

SANGAMO

AGHI

HALF-BRIDGE

1

µF

500

Ω

500

Ω

MECHANICAL

POSITION

INPUT

FULL SCALE

Figure 28. Alternate Half-Bridge Circuit