Adafruit MCP3421 18-Bit ADC – STEMMA QT / Qwiic

The Adafruit MCP3421 18-bit ADC is a simple, inexpensive, and easy-to-use 18-bit, 240 SPS, single-channel ADC with an I2C interface that can run up to 3.4MHz clock rate. A perfect component whenever you need an ADC that has differential inputs, adjustable gain, and a built-in precision/low-drift reference voltage.

One of the trade-offs with getting 18-bit precision is that the ADC is not going to be very fast: you can configure the chip to do a faster 12-bit conversion at 240 SPS, but at 18-bits it slows down to 3.5 SPS.
That’s because the way a sigma-delta ADC works, it ‘guesses’ the analogue voltage and uses a comparator to determine whether the input is higher or lower. Each ‘guess’ takes an extra step, and thus
halves the throughput, so 12-bit is 240 SPS, and 14-bit is 1/4 (2-bit) slower, 60 SPS. Ditto 16-bit is 1/4 slower, 15 SPS, and finally, 18-bit is 3.75 SPS.

The MCP3421 is already set up for differential inputs, which means that you can read positive or negative differences between the two inputs, as long as both signals are between 0 and 2.048V. This means it’s not going to be great for reading stuff like potentiometers, where you have a single-end reading referenced to ground, and you want to read the full range from 0 to Vcc. It is great, however, for reading sensors like strain gauges, pressure sensors, or thermocouples.

We have a ready-to-go Arduino library that makes usage simple: select the sample rate/precision you want and one-shot or continuous mode. Then read values over I2C! This sensor has a fixed address, so you may want to use a multiplexor if you want to connect multiple MCP3421s on a single I2C bus.

To get you going fast, we spun up a custom-made PCB in the STEMMA QT form factor, making it easy to interface with. The STEMMA QT connectors on either side are compatible with the SparkFun Qwiic I2C connectors. This allows you to make solderless connections between your development board and the MCP or to chain it with a wide range of other sensors and accessories using a compatible cable.

QT Cable is not included, but we have a variety in the shop.

Comes with a bit of 0.1′ standard header in case you want to use it with a breadboard or perfboard. Four mounting holes for easy attachment.

Specifications

• 18-bit ΔΣ ADC
• Differential Input Operation
• Self Calibration of Internal Offset and Gain Per Conversion
• On-Board Voltage Reference:
  • Accuracy: 2.048V ± 0.05%
  • Drift: 15 ppm/°C
• On-Board Programmable Gain Amplifier (PGA):
  • Gains of 1,2, 4 or 8
• On-Board Oscillator
• INL: 10 ppm of FSR (FSR = 4.096V/PGA)
• Programmable Data Rate Options:
  • 3.75 SPS (18 bits)
  • 15 SPS (16 bits)
  • 60 SPS (14 bits)
  • 240 SPS (12 bits)
• One-Shot or Continuous Conversion Options
• Product Dimensions: 25.4mm x 17.8mm x 12.3mm
• Product Weight: 3.0g

Resources

• Primary Guide