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5962-89807012A

Balanced Modulator/Demodulator

Analog Devices 

AD630

[

T

]

B. 200mV/DIV

500␮s/DIV

3

C. 200mV/DIV

T

A. 200mV/DIV

Figure 13. AC Bridge Waveforms (1 V Excitation)

LOCK-IN AMPLIFIER APPLICATIONS

Lock-in amplification is a technique used to separate a small,

narrow-band signal from interfering noise. The lock-in amplifier

acts as a detector and narrow-band filter combined. Very small

signals can be detected in the presence of large amounts of

uncorrelated noise when the frequency and phase of the desired

signal are known.

The lock-in amplifier is basically a synchronous demodulator

followed by a low-pass filter. An important measure of performance

in a lock-in amplifier is the dynamic range of its demodulator.

The schematic diagram of a demonstration circuit which exhibits

the dynamic range of an AD630 as it might be used in a lock-in

amplifier is shown in Figure 14. Figure 15 is an oscilloscope

photo demonstrating the large dynamic range of the AD630.

The photo shows the recovery of a signal modulated at 400 Hz

from a noise signal approximately 100,000 times larger.

CLIPPED

BAND-LIMITED

WHITE NOISE

B 16 5k⍀

AD542 1 2.5k⍀

AD630

15

10k⍀

20 A

13

19

17 2.5k⍀

B

100dB

ATTENUATION

14 10k⍀

A

10

0.1Hz

9

MODULATED

400Hz CARRIER

CARRIER PHASE

REFERENCE

C

100R

AD542

R

100R

C OUTPUT

LOW-PASS

FILTER

Figure 14. Lock-In Amplifier

5V

5V

5s

100

MODULATED SIGNAL (A)

90

(UNATTENUATED)

ATTENUATED SIGNAL

PLUS NOISE (B)

10

0%

5mV

OUTPUT

Figure 15. Lock-In Amplifier Waveforms

The test signal is produced by modulating a 400 Hz carrier with

a 0.1 Hz sine wave. The signals produced, for example, by

chopped radiation (i.e., IR, optical) detectors may have similar

low frequency components. A sinusoidal modulation is used for

clarity of illustration. This signal is produced by a circuit similar

to Figure 9b and is shown in the upper trace of Figure 15. It is

attenuated 100,000 times normalized to the output, B, of the

summing amplifier. A noise signal that might represent, for

example, background and detector noise in the chopped radia-

tion case, is added to the modulated signal by the summing

amplifier. This signal is simply band limited clipped white noise.

Figure 15 shows the sum of attenuated signal plus noise in the

center trace. This combined signal is demodulated synchro-

nously using phase information derived from the modulator,

and the result is low-pass filtered using a 2-pole simple filter

which also provides a gain of 100 to the output. This recovered

signal is the lower trace of Figure 15.

The combined modulated signal and interfering noise used for

this illustration is similar to the signals often requiring a lock-in

amplifier for detection. The precision input performance of the

AD630 provides more than 100 dB of signal range and its

dynamic response permits it to be used with carrier frequencies

more than two orders of magnitude higher than in this example.

A more sophisticated low-pass output filter will aid in rejecting

wider bandwidth interference.

–10–

REV. E

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