I have completed the schematic for the power supply and filtering system. I have a first pass at the amplifier chain design, at least for single endend and differential voltage inputs.
Many lock in amplifiers include a transimpedance amplifier to make current measurements, for instance with photodiodes. From looking at some demonstrations on youtube (link below), many of these units are capable of femptoAmp sensitivities. I'm currently shopping for a transimpedance amplifier design with sufficiently low noise to support these measurements.
I am also looking for a PLL system for the demodulator. I'm using the ADA2200, which requires a reference clock at 64x the modulating frequency. So I'm looking for a PLL that will lock at low frequency; Ideally down to 10Hz or less for maximum flexibility, but 100Hz is probably more realistic due to flicker noise.