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5962-8980701RA Datasheet(PDF) 10 Page - Analog Devices |
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5962-8980701RA Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page ![]() REV. E AD630 –10– 500 s/DIV B. 200mV/DIV C. 200mV/DIV 3 [T] 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. 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 14. Lock-In Amplifier 100 90 10 0% 5V 5V 5s 5mV MODULATED SIGNAL (A) (UNATTENUATED) ATTENUATED SIGNAL PLUS NOISE (B) 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. |
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