AD630
20V
0V
100
90
5V
200 s
BRIDGE EXCITATION
(20V/DIV) (A)
AMPLIFIED BRIDGE
OUTPUT (5V/DIV) (B)
DEMODULATED BRIDGE
OUTPUT (5V/DIV) (C)
FILTER OUTPUT (2V/DIV) (D)
0V
0V
10
0%
5V
0V
2V
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.
CLIPPED
BAND-LIMITED
WHITE NOISE
C
B 16
5k
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.
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
20-Lead Ceramic DIP (D-20)
AD630
15
10k
A
B
100R
AD542
1 2.5k
20
19
AD542
13
R
17 2.5k
100dB
ATTENUATION
A
10
0.1Hz
9
MODULATED
CARRIER
400Hz
PHASE
CARRIER
REFERENCE
14 10k
100R
C
LOW PASS
FILTER
OUTPUT
20-Lead Plastic DIP (N-20)
Figure 23. Lock-In Amplifier
PRINTED IN U.S.A.
0.015 (0.38)
MIN
0.028 (0.71)
0.022 (0.56)
1
0.011 (0.28)
0.007 (0.18)
R TYP
0.075
(1.91)
REF
0.088 (2.24)
0.054 (1.37)
0.055 (1.40)
0.045 (1.14)
13
BOTTOM
VIEW
9
0.150
(3.81)
BSC
0.050
(1.27)
BSC
45
°
TYP
5V
100
90
5V
5s
MODULATED SIGNAL (A)
(UNATTENUATED)
ATTENUATED SIGNAL
PLUS NOISE (B)
0.100 (2.54)
0.064 (1.63)
LCC (E-20A)
0.075
(1.91)
REF
0.200 (5.08)
BSC
0.100
(2.54)
BSC
20
10
0%
OUTPUT
5mV
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
–8–
0.358 (9.09)
0.342 (8.69)
SQ
0.358
(9.09)
MAX
SQ
0.095 (2.41)
0.075 (1.90)
REV. C
C00784–0–6/00 (rev. C)
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.
AD [ ANALOG DEVICES ]
