For microcontrollers without an analog-to-digital converter or when you want a higher-precision ADC, 16-Bit ADC - 4 Channel with Programmable Gain Amplifier - ADS1115 provides 16-bit precision at 860 samples/second over I2C. The chip can be configured as 4 single-ended input channels, or two differential channels. It even includes a programmable gain amplifier, up to x16, to help boost up smaller single/differential signals to the full range. 16-Bit ADC - 4 Channel with Programmable Gain Amplifier - ADS1115 can run from 2V to 5V power/logic, can measure a large range of signals and it is very easy to use. It is a great general purpose 16 bit converter.
ADS1115 is fairly small so it comes on a breakout board with ferrites to keep the AVDD and AGND quiet. Interfacing is done via I2C. The address can be changed to one of four options so you can have up to 4 16-Bit ADC - 4 Channel with Programmable Gain Amplifier - ADS1115 connected on a single 2-wire I2C bus for 16 single ended inputs.
Arduino Library and Example Code (Adafruit): https://github.com/adafruit/Adafruit_ADS1X15
Raspberry Pi Library and Example Code (Adafruit): https://github.com/adafruit/Adafruit-Raspberry-Pi-Python-Code
Adafruit Web Site: https://www.adafruit.com/product/1085