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The datasheet of the AD630 mentions that the device can be used as a Precision Phase Comparator or as a Lock-in Amplifier.
What's the difference? They both seem to map the amplitude of a modulated signal to around DC. That is, the output of both circuits is the demodulated signal. (Assuming the reference signal is the modulated signal)
Why would I want to pick one circuit over the other?
The same circuit may be used for different purposes depending on what inputs you apply. And you pick depending on what you want to achieve. Amplifiers are very versatile components and you can achieve quite different goals with the same component.
Assuming the reference signal is the modulated signal is one of possible inputs and then you use the circuit as a demodulator - you get the demodulated signal on output - a simple receiver of modulated signal.
If you pass two identical, or very similar signals (modulated or not) which are phase-shifted, you can treat the output as measurement of the phase shift - Phase Comparator - measuring system delay, e.g. round-trip time of echo vs reference in distance measurements using ultrasound sensors.
If you apply a noisy signal and the reference input gets attenuated noise plus carrier, you're getting a demodulator that can deal with the noise - just like the first case but filtering high-frequency noise away.
