An esp-idf component for interfacing with the NAU7802 24-bit ADC, especially as packaged on e.g. the Adafruit NAU7802 and SparkFun Qwiic Scale. This is similar to the HX711, and usually used for working with load cells.
The NAU7802 is available as a discrete component in both KGI (PDIP16L through-hole) and SGI (16L SOP SMD) form factors. It operates over I²C. It can accept between 2.7V and 5.5V, and operates between -40C and 85C.
This is present in the Component Registry as dankamongmen/nau7802.
The NAU7802 exposes a two-channel differential 24-bit ADC over an I²C-compatible wire interface (plus optional extensions).
A load cell will connect to the NAU7802 using four wires: A+/A- and E+/E-. The latter are outputs from the NAU7802 carrying the excitation voltage to the load cell, and ought be as constant as possible. E- will usually be connected to ground, and E+ to the NAU7802's REFP or AVDD pins. The former are outputs from the load cell, and connect to VinXP and VinXN, respectively.
A differential ADC measures the difference between two signals. Though both the signals are positive voltages, the result will be negative if the N line is higher than the P line (though the lines are "negative" and "positive", they are carrying positive signals). The smallest negative number corresponds to a 0 on the positive line and a maximum signal on the negative line. The largest positive number corresponds to a maximum signal on the positive line and a 0 on the negative line. A 0 corresponds to equal signals of any value.
The primary advantage of a properly routed differential signal is resistance to path noise and common-mode noise, which ought affect both signals equally. Other benefits include easy handling of large DC offsets.
A load cell typically provides its ground to the negative input, and is thus a single-ended sensor. A single-ended sensor ought never generate a negative value in normal use. Zero load will ideally result in a zero signal. One effect is that our 24-bit differential ADC immediately loses a bit, since the negative dynamic range is unused.
The NAU7802 presents two channels, but switching between them is an expensive
operation (including an internal calibration). The channels have different
input capacitance. If you only need one channel, a capacitor to ground can tie
together Vin2N and Vin2P to eliminate some noise; suggested values are 330pF
for 3.3V AVDD and 680pF for 4.5V AVDD. If this is done, be sure to call
nau7802_set_pga_cap(i2ch, true).
The NAU7802 can accept between 2.7V and 5.5V for its digital input DVDD. It
should generally be the same power source as your MCU (e.g. 3.3V for ESP32).
AVDD powers the analog components, and must not exceed DVDD. It can either
come from an external power source, or be converted from DVDD using an
internal LDO. By default, the NAU7802 expects a voltage on AVDD. To instead
use the LDO, call nau7802_enable_ldo() with the greatest nau7802_ldo_level
less than your DVDD (e.g. NAU7802_LDO_30V when powered by a 3.3V DVDD).
pga_ldomode sets the LDOMODE bit of the PGA register, allowing use
of a higher ESR capacitor…but I'm not quite sure what capacitor it refers to.