22-bit A/D Converters

When accuracy matters more than speed, the MCP3550 series of analogue-to-digital converters from Microchip is worth a look. They are ideal for accurately measuring DC voltages that only change slowly over time, offering an extremely high resolution of 22 bits while only drawing a paltry 150 µA from a 5 V power supply. With the addition of a low power voltage reference such as the MAX6520 as shown in this article we have an ultra-precise analogue-to-digital converter with a total current consumption of around 0.2 mA, using only a minimum number of components.

http://ww1.microchip.com/downloads/en/DeviceDoc/21950d.pdf
http://datasheets.maxim-ic.com/en/ds/MAX6520.pdf