2 Theory

The goal of oversampling is to increase ENOB by reducing the noise observed in the signal. Oversampling performs multiple conversions on the same input signal and accumulates the digital values to attain an ENOB higher than the inherent ENOB of the ADC. The precision of the result increases, depending on how much oversampling takes place. This accuracy can be demonstrated by measuring a varying input signal to determine the major frequency of the signal. The amount of oversampling possible is theoretically limited to the data width of the variable used to store the conversion result. For instance, a 16-bit result word limits you to 16 × oversampling on a 12-bit ADC, with a maximum accumulated value of 65535.

In addition to data size constraints, the amount of oversampling is limited by the relationship between the throughput of the ADC and the fundamental frequency of the input signal, as the number of oversampled conversions per second cannot fall below the Nyquist rate. This also means that the oversampling factor is limited by the control loop frequency needed to achieve the system performance requirements.

The size limit occurs because oversampling accumulates the results, which invariably requires more memory than the original result because there can be an overflow from the addition. The accumulated values are not averaged since this effectively removes the additional precision that is obtained. As such, averaging maintains the size of the stored result and the reduced noise, but this does not affect the observed ENOB of the result to any significant degree.

Oversampling with accumulation improves noise reduction in the final value obtained, but the ENOB does not increase as much if there is significant noise affecting the signal. There are several board layout guidelines that, if followed, can help to minimize significant sources of noise in analog signals for ADC conversion. These include:

• Verifying no signal crossing between analog and digital signals
• Having separate layers for analog and digital signals
• Having a dedicated return ground for analog signals that are not shared with digital
• Isolating the analog region from the digital region

For more details about good hardware design for C2000 ADCs, see Section 4.

A Fast Fourier Transform (FFT) is used in this document to process the oversampled ADC results stored in memory. The FFT plot gives us a view of the signal noise and harmonic distortions that affect the observed major frequency, and as such diminish the ENOB. These values are quantified from the FFT data and used to compute an approximate ENOB value. For the purpose of testing, the FFT was computed on ADC data exported from RAM. Before the ADC results have an FFT performed on them, windo