AD630ADZ datasheet(18/20 Pages) AD

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AD630

Data Sheet

Rev. G | Page 18 of 20

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 35. Figure 34 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.

Figure 34. 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 (that is, 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 28 and is shown in the upper trace of Figure 34. 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 radiation

case, is added to the modulated signal by the summing amplifier.

This signal is simply band limited, clipped white noise. Figure 34

shows the sum of attenuated signal plus noise in the center

trace. This combined signal is demodulated synchronously

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 34.

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 aids in rejecting

wider bandwidth interference.

Figure 35. Lock-In Amplifier

100

5mV

MODULATED SIGNAL (A)

(UNATTENUATED)

ATTENUATED SIGNAL

PLUS NOISE (B)

OUTPUT

AD630

R100R

CARRIER

PHASE

REFERENCE

CLIPPED

BAND LIMITED

WHITE NOISE

Ω

0.1Hz

MODULATED

400Hz

CARRIER

2.5kΩ

–

10kΩ

100R

ATTENUATOR

(100dB)

–

–VS

RINA

CH A+

CH A–

CH B+

CH B–

SELB

SELA

+VS

OUTPUT

VOUT

COMP

CHANNEL

STATUS B

2.5kΩ

RINB

–

RB 10kΩ

AD711

STAR

GROUND

STAR

GROUND

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