I've been letting this project lie fallow for the last couple weeks. I intend on getting a dedicated lock in amplifier board built. here is the todo list for that effort.

• Characterize components:
• Put up a resistor / pot bridge, see what sensitivity we get and whatnot…
• Just do the lock in portion.
• Add back in pre-amp - don’t worry ‘bout rail protection for today - monitor
• Change midpoint resistors!
• See if ad8429 works with shorted Rg for 10,000x gain? Should be like 0.6 ohm (check math)?
• Possible re-design of input amp?
• Work noise budget!
• Test point selection
• 100 mil header
• 0603 probe pads - https://www.digikey.com/product-detail/en/te-connectivity-amp-connectors/RCT-0C/A106144CT-ND/3477801
• Get rid of ground pigtails somehow?
• Mmcx connectors - $$$
• Connector selection
• BNC - edge mount
• SMA - edge mount
• 100 mil headers
• Guard traces
• Possibly a can? Pick an optional can and footprint.
• https://www.digikey.com/product-detail/en/leader-tech-inc/SMS-201C/1798-1176-ND/7203462
• Transimpedance amp w/ switches for single ended, differential, and current inputs
• Component selection
• Switches - spdt
• Switched gain
• common numbers for measurement (1x one of the amps)
• Switch selection
• 2xN 100 mil header jumpers
• N pole N throw slide switch - $$$? Real estate?
• DIP
• amp cascade - LNA + “normal” amp
• Nominal goal for 1 nV sensitivity with long enough time base; 1nV -> ~1V
• 1e9 v/v system gain.
• Ad8429 does 1e4 v/v, leaving 1e5 remaining
• Run noise budget trade; friis.
• 1e2 + 1e3 cascade (order)
• 1e4 , 1e1 cascade?
• Run LNA at lower gain (i.e. 1e3 for each stage?)
• Datasheet + friis to make trade
• Multiple 1e3 stages would be more user friendly. Just a chain of amps w/ a single switch to enable (1e3 v/v) or disable (1 v/v).
• Noise budget for additional esd protection at input?
• Diode noise acceptable?
• Or just not do it and label as esd sensitive? Cheap enough components to blow...
• LNA selection
• Reason not to pick AD8429?
• Secondary amp selection
• PLL w/ divide for lock in?
• F, 2F modes
• PLL lock / unlock indicator LED
• Component selection, again...
• Line filter at input - concerns with saturation
• Switched enable - dip or jumper
• Power supply
• LDO selection, +5, -5, +3v3? Unless others necessary
• 7805 serries
• others
• Line filter - maybe?
• Pi filters from current design should be good.
• Beef up bulk caps
• Move indicator diodes to clean side of supply
• Reverse bias protection w/ fets - clean side / dirty side?
• Supply connector w/ bias protection
• Output integration / lpf
• Switched time constant / output buffer
• Long time constants (~10s) desireable
• Output monitor connectors
• Amplified signal
• Demodulated signal, I/Q components
• LPF output
• Phase shifters - include or not?
• How to even implement?
• Indicators, dials, connectors, etc. start pretending at being a “real instrument”