Traditional lock-in amplifiers implement PSD through an analog multiplier. But this kind of method that realizes the coherent modulation with the simulation technology has many flaws which will limit the precision of the phase sensitive detector greatly and introduce a lot of background noise. The digital lock-in amplifier does not produce this problem, and has a very high performance.
Embodied as follows:
In addition to the previous stage, the PSD module, low-pass filter and reference circuit of the digital lock-in amplifier have no temperature drift. The deviation of the output is caused mainly by DA conversion and bits accuracy. Meanwhile, the PSD module, low-pass filter and reference circuit of the analog lock-in amplifier all have temperature drift which will introduce serious errors, making a certain error between output and actual results (that is, systematic errors, and it is often with uncertainty).
Digital lock-in amplifier will not introduce noise through algorithm calculation and is basically free from interference from the external environment. However, PS and filter build by analog circuits will introduce various kinds of background noise because of limited usage of electronic components. Beside, the ambient noise is also coupled to the analog circuit, and the result of the coherent modulation is incorrect when the magnitude of the background noise is close to the signal or larger than the signal. The dynamic reserve of the phase-sensitive detector realized by analog technology is basically limited to below 60 dB. The dynamic reserve of the PSD realized by digital technology can reach 90dB or more. For example, the dynamic reserve of OE1022 is up to 100dB.
As devices evolve, the reference signal of digital lock-in amplifier, such as OE1022 can achieve 24-bit or higher bit-widths. In PSD module, suppression of harmonic components enables distortion of -90db or lower. In addition, due to the usage of digital filters, it will not introduce harmonic distortion caused by op amps.
the digital filter structure is simple and easy to debug. Since the front and rear filter have no effect to each single-stage filter, the low-pass filter can be made very steep and can be freely selected. The filter of OE1022 is divided into 6,12,18,24dB / oct and time constants extrend from 10us to 3000s. The center frequency of analog filter will change with different devices characteristics and temperature drift, which make a hard debug and introduce some of the error.
For low-frequency signals, the general filter is powerless or with little effect after filt out the AC component. Then you need to use synchronous filter, which is equivalent to a very good low-pass filter, to average data in a period. It requires a large external circuits to realize a good synchronous analog filter. It's an unwise choice from cost so that its low frequency signal measurement is usually not too low. Meanwhile, digital filter can store a huge amount of data through its large capacity. Now OE1022 can achieve the time constant of 3000s, which is the base of 1mHz frequency measurement accuracy.