Harmonic Distortion and Adc

The influence of different ADC nonlinearities on the measurement of ADC intermodulation products is discussed. It is shown that the amplitudes of harmonics and intermodulation products can be predicted only if the low order nonlinearity of the transfer curve is dominant and if a sufficient level of input noise or dither is used. 1 INTRODUCTION The ADC nonlinearities produce persistent sine waves error signals that affect more severely ADC performance than noise. The influence of some nonlinearities, such as quantization and DNL can be minimized by using dither or input noise that spread the spectrum of the corresponding error signals and so transfer the persistent sine waves power to noise. However, the influence of others, such as those of the low order nonlinearity of the transfer curve or of the dynamic behavior of the S/H circuit remains. The sine waves error signals can be divided into two groups: mixing products of the input signal (harmonics and intermodulation products) and mixing products between the input and the sampling frequency (spurious). The harmonics and intermodulation products could be considered as low order mixing products of the input produced by the nonlinear smoothed transfer curve. Being so not only the frequency, but also the amplitude could be predicted. The mth-order mixing product would change according to the mth-order law about its frequency and amplitude. If this is valid, the ADC's low order non-linearity could be described as in the case of analog circuits by intercept points or by the polynomial expression that describe the smoothed transfer curve [1,2]. Other attempts were made to compute the INL from harmonics amplitude [3]. In this paper we present numerical simulation and experimental results corresponding to preliminary work in this field. Our aim is to present our experience and open the discussion about the proper definition of harmonics and spurious. The main question is: when the signal changes only its frequency according to the mth-order law and its amplitude is random, is it an harmonic or a spurious component? 2 THEORY The smoothed ADC's transfer curve or the nonlinearity of an analog IC can be expressed by the low order polynomial expression y =a 0 + a 1 x + a 2 x 2 + a 3 x 3 + ....+a n x n , (1) where y is the equivalent the output signal, x the input signal and n the order of the circuit nonlinearity. Such …