Linear Characteristics of ADC using Raspberry Pi

Abstract

Our intention is to measure the linear characteristics of an ADC using a Raspberry Pi. The sampling of analog data and subsequent conversion or quantization is performed by the ADC. Quantization is a process where the analog values belonging to a larger set are rounded and mapped to a smaller set of countable values. By sampling and quantizing analog voltage data, we can verify if the characteristics between the analog and digital data is indeed linear.

Introduction

The aim in this lab is to use a Raspberry Pi with ADC to verify the linear characteristics of an ADC (MCP3008). The code is written in ’Python’ language. A potentiometer is connected to vary voltage values between 0 and 3.3V. A 10-bit ADC is used to sample this analog data. This ADC is SPI-based. Our board is powered by a (5V, 2.5A) power supply. We connect to the Pi using an ethernet cable. We log into the Pi using ssh. The compilation and execution of code is done using the serial terminal. In this paper, we will be explaining the design methodology that was taken, the design implementation, and how the testing and verification was done

Algorithm

1. Functions for SPI initialization, Set the MOSI, MISO, SCK, CS
2. Create a function readadc with parameters (adcnum, clockpin, mosipin, misopin, cspin)
3. This function will read from DOUT and DIN pins from ADC and return adcout values i.e the sampled and quantized value.
4. In a while loop, we keep calling readadc function and convert the quantized value to understandable voltage values when the adcout value has changed i.e the pot has been moved
5. The adcout value is to be to convert from a 10-bit (0-1023) range to a (0-3.3V) range
6. We then multiply by 3.3 and divide by 1023 to get the final voltage value

Testing and Verification

This section deals with the hardware and software testing and verification:

A. Hardware Testing

Testing of the hardware components and connections are done using a multimeter. When the Raspberry Pi is powered correctly the red and green LEDs will turn on.

B. Software Testing

By writing simple GPIO testing programs, we perform input and output testing of GPIO.

C. Verification

1. When the ADC code is run, it displays the voltage values (0 to 3.3V) along with its 10-bit range value (0 to 1023)
2. The difference between the actual and expected value gives us the error