The tiny el cheapo ADC for the W is
based on the mighty CM109 from Taiwan
It's rated for an 83dB SNR. It wasn't a warm & fuzzy way to close this chapter, so took some more notes on the AK4524.
The AK4524 works at 3.3V, despite being rated for 5V. The W causes a 2ms, 40mV drop every 100ms in the power supply. The W runs on 4.2V.
The AK4524 + AD8604 preamp suck 25mA when converting. With the AK4524 off, the analog section only uses 3.5mA. A simple RC filter with 75R & 2000uF knocks the noise to a reasonable level while knocking the voltage from 5V to 3.2V. A 35R + 2000F . Being the junk circuit it is, it could just use a separate 4.2V battery for the analog section & a 4.2V battery for the W. The W runs on 3.5V if nothing needs its USB power.
The 70R + 1470uF filter. LTSpice says the output ripple is 5uV.
The 35R + 1470uF filter. LTSpice says the output ripple is 1.1mV. There were some notes about cascading filters & inductors on the goog, but this topic is where driving LCDs was 7 years ago, completely obscure.