Blog #2: Getting Started with MAX40080 Current Sense Amplifier

The working principle of a current sensing amplifier is based on Ohm's law. When load current flows through a shunt resistor (Rshunt) present on inputs, it generates a voltage drop called V_SENSE. This voltage is generally small to limit power dissipation losses. V_SENSE is then amplified with an internal current sense amplifier. The resulting output voltage (V_OUT) is a voltage that is proportional to the Ioad current. It can then be processed with an ADC (analog-to-digital converter).

The MAX40080 Current Sense Amplifier

The MAX40080 is a high-precision, fast-response, bi-directional current-sense amplifier with digital output and a very wide input common-mode range from -0.1V (ground sensing) to 36V.

The device features an ultra-low 5μV input offset voltage and a very low 0.2% gain error. The low input offset voltage is especially important because it allows using a small sense resistor, thus saving power dissipation, but at the same time not compromising the measurement accuracy. The device also features a programmable input sensing range between ±10mV and ±50mV (or programmable input gain between 125V/V and 25V/V) which is very useful to enhance accuracy at low current.

The device includes an analog-to-digital converter with a programmable sample rate and 12-bit resolution (13-bit including sign bit for current measurement) and features an I²C compliant and SMBus compatible interface.

The device features a wake-up current threshold and auto-shutdown mode when the I²C is inactive. Both these features are designed to minimize power consumption.

The device is available in a small 12-pin WLP (and also a 12-pin TDFN) and is specified over the -40°C to +125°C extended operating temperature range.

Current 6 Click

Current 6 Click is a compact add-on board providing a precise and accurate current sensing solution. This board features the MAX40080, a current-sense amplifier from Analog Devices discussed above. This Click board delivers higher performance to industrial control and automation applications, load and power supplies monitoring, telecom equipment, and many more.

Current 6 Click as its foundation uses the MAX40080, a high-precision, fast sample-rate digital current-sense amplifier from Analog Devices. The MAX40080 measures current and common-mode voltage ranging from -0.1V to 36V and converts the data into digital form through an I2C-compatible two-wire serial interface allowing access to conversion results. Also, setting the input voltage sense using the I2C register, ±50mV or ±10mV, will allow one to select two measuring ranges from 0A to 1A or from 0A to 5A.

This Click board communicates with MCU using the standard I2C 2-Wire interface for configuring and checking the device's status. Standard I2C commands allow reading the data and configuring other operating characteristics. While reading the current/voltage registers, any measured current and voltage changes are ignored until the read is completed. The current/voltage register is updated for the new measurement upon completion of the read operation.

Schematic for Current 6 Click

Getting Started with Current 6 Click

I received a Pi 3 click shield/Expansion board and current 6 click for free from Element14 as part of this design challenge but unfortunately, I did not receive Raspberry Pi 3. I checked the user manual for Pi 3 click shield and found that it works with Raspberry Pi 2 B also. Fortunately, I have a Pi 2 in my parts bag. So, In this blog, I am going to use the current sense 6 click with my Raspberry Pi 2. MikroElektronika does not provide any library for the current sense 6 click for python or Raspberry Pi environment. So, I will use python library written by misaz. The library is here (https://github.com/misaz/MAX40080-PythonLibrary). Mr. misaz written an excellent blog on getting started with the current 6 click with Raspberry pi. You can find it here. 

The connection for measuring current will be like in the following image. 

For testing purposes, I am using a 5W, 33ohm resistor with a power supply. I used high-current jumper wires for connecting the sensor to the measuring circuit. If your jumper wires can not handle much current you can get the wrong value for a significant voltage drop in the jumper wire. It is very important for high current measurement. Traditional breadboard jumper wires can handle around 1 A without a significant voltage drop. 

Now, enable i2c from the raspberry pi configuration. You can use the following command in the terminal to go to the configuration option.

sudo raspi-config

The current 6 click board should be enabled by providing high signal to the enable pin that is connected to CS pin of the mikroBUS socket. If you plug the current 6 click board into socket 1 of the expansion shield the CS pin will be connected to the GPIO 8 of the Raspberry Pi. So, you have to make this pin high to enable the current 6 click. The GPIO 8 Pin can be set as HIGH using the echo command from the terminal:

echo 8 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio8/direction
echo 1 > /sys/class/gpio/gpio8/value

You have to use GPIO 7 instead of 8 if you use 2nd slot. 

Now you can detect your device using the following command from the terminal

i2cdetect -y 1

If you can detect your i2c device successfully then download the following two python libraries in your Pi. The first one is for i2c communication and the second one is for making the plot. 

pip3 install smbus 
pip3 install matplotlib

After downloading the above libraries create a python file and add the following code. You can create the file directly in your Raspberry Pi or if you access Raspberry Pi using a serial terminal it is better to create the python file on your pc and then transfer the file to the raspberry pi using any FTP client like WinSCP. I created the python file named current_plot.py. 

import RPi.GPIO as GPIO
import smbus
import matplotlib.pyplot as plt
import time

GPIO.setwarnings(False)
GPIO.setmode(GPIO.BOARD)
GPIO.setup(8,GPIO.OUT)
GPIO.output(8,GPIO.HIGH)

max40080_addr = 0x21

bus = smbus.SMBus(1) 

# set mode from standby to active
bus.write_i2c_block_data(max40080_addr, 0x00, [0x63, 0x00, 0x7D])

time.sleep(.1)

# check configuration
value = bus.read_i2c_block_data(max40080_addr, 0x00, 3)
print("cfg reg =", value)

# read measurement
value = bus.read_i2c_block_data(max40080_addr, 0x0C, 3)
print("data reg =", value)

measurements = []
for i in range(100):
    value = bus.read_i2c_block_data(max40080_addr, 0x0C, 3)

    filtered_flag = ((value[1] & 0x7F) << 8) | value[0]
    sign_ext = ((filtered_flag & 0x4000) << 1) | filtered_flag
    bytes_after = [sign_ext & 0xFF, (sign_ext & 0xFF00) >> 8]
    measured_value = int.from_bytes(bytes_after, "little", signed="True")

    measurements.append(measured_value)
    time.sleep(0.001)
    
print("Average:", sum(measurements) / len(measurements))
plt.plot(measurements)
plt.ylim(ymin=0)
plt.show()

Now, run the python file using the following command:

python3 current_plot.py

You will see a graph that looks like as the image below.

In the next blog, I will measure the high side and low side current for a circuit and compare the result.