Oversampling with averaging to increase ADC resolution

Franco Contadini, Maxim

March 01, 2010

Franco Contadini, MaximMarch 01, 2010

When considering the resolution required for an A/D converter (ADC) integrated in a microcontroller (MCU), embedded systems designers must balance cost and performance. Higher ADC resolution implies higher-cost MCUs, but in some cases you can use other features in the MCU to enhance the ADC performance via software. That approach lets you achieve higher resolution using an inexpensive integrated ADC. Here's how to use of oversampling to achieve extra bits of resolution for an ADC integrated in an MCU.

We start by examining the frequency-domain transfer function of a multibit ADC operating on a sinewave input signal. The ADC samples this input at a frequency Fs, which (according to Nyquist theory) must be at least twice the input-signal bandwidth. An FFT analysis (left graph of Figure 1) shows a single tone with lots of random noise (known as quantization noise) extending from DC to Fs/2.

Click on image to enlarge.

You obtain the signal-to-noise ratio (SNR) by dividing the amplitude of the fundamental by the RMS sum of all frequencies containing noise. For an N-bit ADC whose peak input voltage equals the ADC's full-scale input voltage, the maximum SNR equals 6.02N + 1.76dB.

Consider now the same example, but with sampling frequency increased by an oversampling ratio of k, in other words, to kFs. An FFT analysis shows that the noise floor has fallen and the SNR is the same, but noise energy has been spread over a wider frequency range (right graph of Figure 1).

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