MCP3421 Weight Scale
Created: Aug 16, 2016
No description available.
This reference design features the capabilities of the MCP3421 device. The MCP3421 is an 18-bit ADC that supports I2C protocol which makes it easy to be interfaced with microcontrollers that also support I2C communication. The device has a very low power consumption. It runs with a 2.7-5.5V power supply and consumes a maximum supply current of 190uA during conversion time. During standby mode, the device only consumes 0.5uA of current. The MCP3421 device performs conversion at rates of 3.75, 15, 60, or 240 samples per second (SPS) depending on the user controllable configuration bit settings that can be accessed through its I2C interface.
This MCP3421 weight scale uses an AMA-2 load cell, MCP6V07 op amps, PIC18F4550 microcontroller, and CG1626-SGR1 16X2 LCD to weigh loads up to 2Kg. The load cell used in this design can measure up to 2Kg with 1mV/V sensitivity. The output of the load cell is connected to the screw terminals J18 and J19. The MCP3421, with its maximum gain setting, can detect an input signal of as low as 2uV. It is important to control the noise level when measuring small signal to obtain an accurate reading. That is why the design utilizes the MCP6V07 op amps since it has an offset voltage that is less than +/-2uV and drifts less than +/-50nV/C. The MCP6V07 device amplifies the output of the load cell. The MCP3421 converts the analog voltage from the MCP6V07 to digital and sends it to the PIC18F4550 microcontroller over the I2C bus.
The PIC18F4550 microcontroller, aside from handling communication with the MCP3421 ADC, it also controls the LCD. The LCD displays the weight of the load. Aside from displaying the weight of the load, the LCD can also be used to display configuration set through switches S2, S3, and S4. The switch S2 is used to change the display from “Weight” to “ADC code”, “ADC output”, or “Calibration”. The switch S3 is used to configure the gain of the MCP3421. Easy zero calibration of the weight scale can be done by pressing S4. The PIC18F4550 microcontroller can also communicate with a PC through the mini USB port. Microchip provides a PC Graphical User Interface (GUI) wherein the user can monitor the reading of the weight scale and calibrate the MCP3421 device for more accurate reading.