Description

This is a simple but versatile Feather compatible board. It uses an AD7794, a very robust 6 channel 24 bit ADC. It's a relatively slow converter (470 Hz Max), but this board is really suited for lower power, monitoring type applications and can consume as little as 2 μA in standby. It has an on-board 2.5 V regulator for excitation voltage and is designed primarily to read load cells and pressure gauges. However, it can also be used to read any ratiometric sensor (such as potentiometers), thermocouples, or any 0 - 3.3 V signal.

Update - New Revision!

I have added 1 MOhm bias resistors to the bottom of the board to make easier/possible to read thermocouples. While the AD7794 has an on-board bias voltage generator, it is only available on channels 1,2 & 3. It also seems to be pretty noisy for some reason. Now you can a read a thermocouple on any channel by soldering the 2 jumpers closed to connect the resistors. (Bias resistors are needed to shift the output of a thermocouple to 1/2 the common mode range of the ADC.)

Specifications

• RMS noise down to 40 nV at lowest data rate (4.17 Hz)
• Power-down current: 1 μA maximum (AD7794 chip)
• Low noise, programmable gain, instrumentation amplifier
• Band gap reference with 4 ppm/°C drift typical
• Simultaneous 50 Hz/60 Hz rejection
• Update rate: 4.17 Hz to 470 Hz (single channel)
• Six differential analog inputs
• 4 pin connections with excitation and ground for each channel
• On board 2.5v LDO with enable pin broken out to turn off excitation for low power applications
• 1 MOhm bias resistors to the bottom of the board for reading thermocouples

The board is available with screw terminals, Plug in spring terminal connectors, or pin headers for the sensor connections. All terminals and headers are sold unsoldered by default.

Why did you make it?

I designed this board to make it easier to get high quality sensor readings with open source hardware. I first used this ADC in a wireless strain gauge logger design back in 2010 and I was very happy with it. Recently while revisiting my old design, I ended up creating a FeatherWing style breakout for easy prototyping with different host microcontrollers. I thought others might find the board useful, so after a couple revisions, here we are. The library has been tested to work with M0, 32U4, ESP8266, and ESP32 based Feathers; and with the Teensy 3.2 (in Feather adapter board). The NHB_AD7794 library is also now available through the Arduino Library Manager.

Library Installation

The library can now be installed from the Arduino library manager, just search for "NHB_AD7794". Alternatively you can manually install by cloning the git repo linked to below.