REV. C

–8–

AD630

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

20-Lead Ceramic DIP (D-20)

20-Lead Plastic DIP (N-20)

LCC (E-20A)

0.358

(9.09)

MAX

SQ

0.088 (2.24)

0.054 (1.37)

0.100 (2.54)

0.064 (1.63)

0.358 (9.09)

0.342 (8.69)

SQ

0.075

(1.91)

REF

0.075

(1.91)

REF

0.011 (0.28)

0.007 (0.18)

R TYP

BOTTOM

VIEW

0.015 (0.38)

MIN

1

20

13

9

0.028 (0.71)

0.022 (0.56)

0.095 (2.41)

0.075 (1.90)

0.200 (5.08)

BSC

0.100

(2.54)

BSC

0.055 (1.40)

0.045 (1.14)

45

°

TYP

0.150

(3.81)

BSC

0.050

(1.27)

BSC

100

90

10

0%

5V

20V

5V

200 s

2V

0V

0V

0V

0V

BRIDGE EXCITATION

(20V/DIV) (A)

AMPLIFIED BRIDGE

OUTPUT (5V/DIV) (B)

DEMODULATED BRIDGE

OUTPUT (5V/DIV) (C)

FILTER OUTPUT (2V/DIV) (D)

Figure 22. AC Bridge Waveforms

LOCK-IN AMPLIFIER APPLICATIONS

Lock-in amplification is a technique which is used to separate a

small, narrow band signal from interfering noise. The lock-in

amplifiers 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 perfor-

mance in a lock-in amplifier is the dynamic range of its demodu-

lator. 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 23. Figure 24 is an oscillo-

scope photo showing the recovery of a signal modulated at

400 Hz from a noise signal approximately 100,000 times larger;

a dynamic range of 100 dB.

A

B

10k

100R

C

OUTPUT

LOW PASS

FILTER

A

B

C

R

100R

AD630

10k

5k

2.5k

2.5k

20

19

17

1

16

AD542

13

AD542

14

10

9

CLIPPED

BAND-LIMITED

WHITE NOISE

100dB

ATTENUATION

0.1Hz

MODULATED

400Hz

CARRIER

CARRIER

PHASE

REFERENCE

15

Figure 23. Lock-In Amplifier

100

90

10

0%

5V

5V

5s

5mV

MODULATED SIGNAL (A)

(UNATTENUATED)

ATTENUATED SIGNAL

PLUS NOISE (B)

OUTPUT

Figure 24. 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 (IR, optical, etc.) 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 18b and is shown in the upper trace of Figure 24. It is

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

summing amplifier. A noise signal which 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 24 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 24.

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 dy-

namic 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.