24 bit ADC for Load Cells / Strain gauges, HX711, SPI

Acquiring load cell/strain gauge data with HX711 ADC amplifier

If you are feeling the stress and strain on a gauge then a Wheatstone bridge (wikipedia) and an HX711 ADC amplifier would allow to quantify it. The HX711 is an handy breakout to do the job! The Adafruit HX711 Breakout contains a super-high-resolution 24-Bit differential ADC containing an extra gain circuitry that makes it perfect for measuring strain and load (or other sensors that have four wires that are connected in a Wheatstone bridge arrangement).

the breakout fits an HX711 ADC chip with some support circuitry and a terminal block (6 terminals) used to connect one or two 4-wire sensors. The E- pad connects to ground (often a black wire), the E+ pad connects to the power voltage supplied to the HX711 (often a red wire).

Then A- and A+ pads are used to connect to the differential outputs of the Wheastone bridge (usually white and green wires on strain gauge). If you have a second gauge or voltage you want to measure, you can use the B- and B+ pads. On the A channel, you can select 64x or 128x gain. On the B channel, you have a fixed 32x gain.

When the sensor bends, the slight changes in resistance cause slight changes in voltage, that change is amplified and read by the HX711 ADC for converting the sensor bends into measurement of force or mass. You can use the Adafruit's Arduino library or MCHobby's MicroPython library or CedarGrove's CircuitPython/Python library to configure and read the ADC for fast interfacing.

The HX711 uses a two-wire SPI alike protocol (clock + Data Out). Such amplifier also exists with I2C interface with the NAU7802 breakout (based on the excellent Sparkfun HX711 breakout: incorporating their split analog/digital supply and digital supply filtering).

Adafruit includes some interesting tweaks:

• pre-soldered terminal blocks,
• the second bridge exposed,
• a "Sample Per Second" slide switch (10 and 80 SPS rates).

Technical details

HX711 Specifications:

• Two selectable differential input channels
• On-chip active low noise PGA with gains of 32 (B), and 64 or 128 (A)
• On-chip power supply regulator for load-cell and ADC analog power supply
• On-chip oscillator requiring no external component with optional external crystal
• On-chip power-on-reset
• Simple digital control and serial interface
• Selectable 10SPS or 80SPS output data rate
• Simultaneous 50 and 60Hz supply rejection
• Current consumption including on-chip analog power supply regulator:
• normal operation < 1.5mA, power down < 1uA
• Operation supply voltage range: 2.6 ~ 5.5V

Product Dimensions: 25.5mm x 23.0mm (Height: 12.1mm)

Weight: 3.9 gr