Data Sheet

AD630

Rev. G | Page 17 of 20

LVDT SIGNAL CONDITIONER

Many transducers function by modulating an ac carrier. A

linear variable differential transformer (LVDT) is a transducer

of this type. The amplitude of the output signal corresponds to

core displacement. Figure 31 shows an accurate synchronous

demodulation system, which can be used to produce a dc

voltage that corresponds to the LVDT core position. The

inherent precision and temperature stability of the AD630

reduce demodulator drift to a second-order effect.

Figure 31. LVDT Signal Conditioner

AC BRIDGE

Bridge circuits that use dc excitation are often plagued by

errors caused by thermocouple effects, 1/f noise, dc drifts in the

electronics, and line noise pick-up. One way to get around these

problems is to excite the bridge with an ac waveform, amplify

the bridge output with an ac amplifier, and synchronously

demodulate the resulting signal. The ac phase and amplitude

information from the bridge is recovered as a dc signal at the

output of the synchronous demodulator. The low frequency

system noise, dc drifts, and demodulator noise all get mixed to

the carrier frequency and can be removed by means of a low-

pass filter. Dynamic response of the bridge must be traded off

against the amount of attenuation required to adequately suppress

these residual carrier components in the selection of the filter.

Figure 33 is an example of an ac bridge system with the AD630

used as a synchronous demodulator. The bridge is excited by a

1 V 400 Hz excitation. Trace A in Figure 32 is the amplified bridge

signal. Trace B is the output of the synchronous demodulator

and Trace C is the filtered dc system output.

Figure 32. AC Bridge Waveforms (1 V Excitation)

Figure 33. AC Bridge System

A

B

10kΩ

10kΩ

5kΩ

2.5kΩ

2.5kΩ

C

100kΩ

D

1µF

AD630

±2 DEMODULATOR

AD711

FOLLOWER

B

PHASE

SHIFTER

A

E1000

SCHAEVITZ

LVDT

2.5kHz

2V p-p

SINUSOIDAL

EXCITATION

16

1

14

17

9

10

20

19

12

13

15

500µs/DIV

B. 200mV/DIV

C. 200mV/DIV

3

[T]

T

A. 200mV/DIV

AD8221

REF

+IN

–IN

49.9Ω

350Ω

350Ω

350Ω

350Ω

CH B–

SEL B

CH A–

SEL A

+15V

COMP

AD630AR

12

13

18

14

10

17

16

15

19

20

911

–15V

4.99kΩ

4.99kΩ

4.99kΩ

2µF

2µF

2µF

A

C

B

1V

400Hz