Circuit Python library for using the Analog Devices AD7124-4, 24 bit ADC

The AD7124-4 is a 4 channel, 24 bit, differential ADC (it can also be configured for up to 7 single ended channels). This library is a port of the NHB_AD7124 Arduino library to Circuit Python. The library is currently written in all Python and is a bit of a memory hog as it is, but it gets the job done. I am new to Circuit Python, so I'm sure there is plenty of room for improvement and optimization.

Like the original Arduino library, this was originally written for use with the NHB AD7124 Analog Sensor FeatherWing, but there is no reason it couldn't be used with a raw chip in your own design.

So far I have only tested this library on an ESP32-C3, but it really should work with any architectures that has a working SPI implementation.

WARNING, THIS IS A WORK IN PROGRESS

The init method takes a Chip Select pin and a previously configured SPI object

def __init__(self, csPin, spi, baud = 4000000):

Where

So, to create an instance...

from CircuitPy_AD7124 import NHB_AD7124

adc = NHB_AD7124.Ad7124(cs,spi)

To set the basic global parameters of the ADC we use the setAdcControl(..) method

def set_adc_control(self, mode: int, power_mode: int, ref_en: bool, 
                        clk_sel: int = AD7124_Clk_Internal):        

Where

The AD7124-4 ICs have a feature defined in the datasheet as "Setups". The Setups allow for pre-configuring for different sensor types, independent of which channel they are assigned to. The setups are modeled in the library as an array of setup objects contained within the Ad7124 class. There are 8 individual setups that can be used to hold different configurations. A single setup can be used for multiple channels of the same type.

The two main methods for configuring a setup are set_config(..) and set_filter(..)

def set_config(self, ref_source, gain, bipolar: bool, 
                   burnout = AD7124_Burnout_Off, 
                   exRefV: float = 2.50):

Where...

def set_filter(self, filter, fs, 
                   post_filter = AD7124_PostFilter_NoPost,
                   rej60: bool = False, single_cycle: bool = False):

Where...

We need need to define what inputs are used by each channel and what setup will be used. This is done with the set_channel(..) method.

def set_channel(self, ch: int, setup: int, aiPos: int,
                   aiNeg: int, enable: bool):  

Where...

There are a few different methods to get readings from the ADC. The most straightforward one is read_volts(ch), which returns the ADC reading in Volts. Note: If using an external reference voltage, the voltage must be properly set with the set_config(..) method for this function to work correctly.

def read_volts(self, ch: int):

The read_raw(ch) method is provided for lower level access to the raw ADC counts for the given channel

def read_raw(self, ch: int) -> uint:

There are also a few sensor specific read functions provided for convenience.

The read_FB(ch, vEx, scale_factor) method is for reading full bridge type sensors (load cells, pressure gauges, extensometers, ...). It could also be used for potentiometers as long as you have setup the channel properly. Returns mV/V if scale factor is one (default). The channel read must already be properly configured for reading the sensor.

def read_fb(self, ch: int, vEx: float, scale_factor: float = 1.00):

The read_ic_temp(ch) method reads the temperature sensor embedded in the AD7124. The channel must be properly configured to read the sensor first.

def read_ic_temp(self, ch: int):

Some sensors like wheatstone bridges, potentiometers, and thermistors, require an excitation voltage to read. The NHB Systems AD7124 boards include a 2.5V linear regulator to provide this excitation. The enable pin of the regulator is tied to the PSW pin of the AD7124-4 allowing it to be controlled by software. This allows the regulator to be powered down to save power between readings in long term, low speed logging applications. To enable the regulator we just need to call the setPWRSW() method. If you are using the AD7124-4 in your own design, this method controls PSW pin (on-chip low side power switch), which can be used to control power to external sensors, but is limited to 30 mA.

The setPWRSW(bool) method controls the internal low side switch connected to the PSW pin on the AD7124-4

def setPWRSW(self, enabled):

import time 
import board
import busio
import neopixel
import gc

from CircuitPy_AD7124 import NHB_AD7124

filterSelectBits = 4 #Fast

csPin = board.A0


spi = busio.SPI(board.SCK, MOSI=board.MOSI, MISO=board.MISO)

adc = NHB_AD7124.Ad7124(csPin,spi)


print("Trying to read ID register...")
ID = adc.get_ID()
print(f"ADC chip ID: {hex(ID)}")


# Configure the ADC in full power mode
adc.set_adc_control(NHB_AD7124.AD7124_OpMode_SingleConv,
                    NHB_AD7124.AD7124_FullPower,True)

# Configure Setup 0 for load cells:
# - Use the external reference tied to the excitation voltage (2.5V reg)
# - Set a gain of 128 for a bipolar measurement of +/- 19.53 mv
adc.setup[0].set_config(NHB_AD7124.AD7124_Ref_ExtRef1, NHB_AD7124.AD7124_Gain_128, True)

# Configure Setup 1 for using the internal temperature sensor
# - Use internal reference and a gain of 1
adc.setup[1].set_config(NHB_AD7124.AD7124_Ref_Internal, NHB_AD7124.AD7124_Gain_1, True) #IC Temp



# Set filter type and data rate select bits (defined above)
#  The combination of filter type and filter select bits affects the final
#  output data rate. The SINC3 filter seems to be the best general purpose
#  option. The filter select bits are a number between 1 and 2048. A smaller
#  number will give a faster conversion, but allow more noise. Refer to the
#  datasheet for details, it can get complicated.
adc.setup[0].set_filter(NHB_AD7124.AD7124_Filter_SINC3, filterSelectBits)
adc.setup[1].set_filter(NHB_AD7124.AD7124_Filter_SINC3, filterSelectBits)



# Set channel 0 to use pins AIN0(+)/AIN1(-)
adc.set_channel(0, 0, NHB_AD7124.AD7124_Input_AIN0, NHB_AD7124.AD7124_Input_AIN1, True)

# Set channel 1 to use the internal temperature sensor for the positive input
# and AVSS for the negative
adc.set_channel(1, 1, NHB_AD7124.AD7124_Input_TEMP, NHB_AD7124.AD7124_Input_AVSS, True) #IC Temp


print("Turning ExV on")
adc.setPWRSW(True)
time.sleep(0.2) # No sleep_ms() in CircuitPy


print("Now try to get some readings...")


while True:
    
    lc_reading = adc.read_fb(0, 2.5, 5.00)  # Read full brideg sensor (Chan, Ex V, Scaling)
    #reading = adc.read_volts(0)         # Or use this to just read the voltage
    print(f"Loadcell = {lc_reading:.4f}", end=',')
    
    ic_temp = adc.read_ic_temp(1)       
    print(f" IC Temp = {ic_temp:.2f}")
        
    time.sleep(0.1) # No sleep_ms() in CircuitPy