Guiding Design Goals and Philosophy for Lock in Amplifier:
Similar capabilities to the sr530. This is a common, workhorse instrument that I would love to have on my bench. The big features i’m going for are:
Expense: this should be “affordable”. I don’t have a hard number, but I want these to be maybe $30-$50 ea. in singles.
Extensibility and limitation of features: This is intended to be both a general purpose instrument, and a “first pass” design. This means I’m not going overboard with high expense or difficult LNA designs, crazy filter topologies, etc. for example, I haven’t selected a tuned JFET LNA, or the lmc6001 transimpedance amplifier for this reason. I’m also not adding a lot of microcontroller measurement or display systems. These can be reserved for either a version 2.0 design or external modules.
Specific key features:
nanoVolt sensitivity, single ended or differential.
I’ll be happy if I get within an order of magnitude of these targets. It is acceptable to add an external preamplifier with a better noise figure if I really need it.
Selectable gain from nV to mV
Modulation frequency selectable from 1 Hz to 100kHz. The ADA2200 requires a clock frequency 64X the modulation frequency, so that puts the PLL output at 64Hz to 6.4MHz. This is a weird frequency range to cover. I have not found any existing products that cover this use case that I’m happy with.
“Arbitrary” waveforms (within reason). The sr530 can lock to any input waveform that has sufficient pk-pk range and crosses it’s midpoint twice per cycle. This permits common logic schemes like TTL, as well as “real world” waveforms like a chopper wheel modulated photodiodes.
Since I’m ultimately looking to do photoacoustic spectroscopy, an optical chopper wheel is a very likely modulation scheme. This is a key feature that I want in an end system.
Adjustable bandpass filter around modulation frequency. I get this for free from the ADA2200, so I don’t intend to add dedicated filters, but this would be a good idea for a future version.