Achieving Higher ADC Resolution Using Oversampling

AN1152 Achieving Higher ADC Resolution Using Oversampling THEORY OF OPERATION Author: Jayanth Murthy Madapura Microchip Technology Inc. As previously mentioned, ADCs transform analog signals into digital sample values. Analog signal amplitude is quantized into digital code words with a INTRODUCTION finite word length. This process of quantization introduces noise in the signal called "quantization An Analog-to-Digital Converter (ADC) is an active inter- noise". The smaller the word length, the greater the face between the analog and digital signal chains in an noise introduced. embedded system. An ADC converts analog signals Quantization noise can be reduced by adding more bits into digital signals in electronic systems. The key fea- into the ADC hardware design. This noise can also be ture of an ADC is the accuracy (resolution) it offers. The reduced in software by oversampling the ADC and then higher the desired accuracy, the higher the ADC cost. processing the digital signal. The oversampling ADC Higher ADC accuracy is achieved by designing hard- method and a few associated terms are explained in ware to quantize the analog signal amplitude into the the following sections. digital signal with a higher code-word length. Practical ADCs have finite word lengths. ADC Voltage Resolution To effectively strike a balance between system cost Voltage resolution of an ADC is defined as the ratio of and accuracy, higher conversion accuracy is achieved full scale voltage range to the number of digital levels by oversampling the low-resolution ADC integrated that are accommodated in that range. It is a measure of within a digital signal controller (DSC), and then pro- the accuracy of the ADC. The higher the resolution, the cessing the oversampled digital signal in software higher the number of levels accommodated in the through a digital filter and a decimator. This processing voltage range and, consequently, the lower the scheme, which adds additional bits of accuracy to the ® quantization noise, as shown in Equation 1. DSC, is explored in12-bit ADC conversion in a dsPIC this application note. EQUATION 1: full scale voltage range volts Voltage Resolution 1 Least Significant bit (LSb) value --------------------------------------------------------==------------ N level - 12 where N is the number of bits or the word length