Dynamic Non-linearity of Impulse Digitisers and Its Effects on Impulse Voltage Measurement

Dynamic non-linearity of analogue-to-digital converters (ADCs) used in digitisers may affect the accuracy of impulse measurements. IEC 610831:2001 defines the method for determining dynamic differential non-linearity (DNL) of impulse digitisers and sets the acceptance limit for the DNL of impulse digitisers. However, the errors of impulse parameters that result from a DNL within such a limit are yet to be studied. Here the DNL of several commercial digitisers has been determined using triangular and sinusoidal test voltages. The results show that the difference in dynamic DNL values obtained with the two types of test voltages was negligible compared with the acceptance limit set in IEC 61083 1:2001. A new parameter, the dynamic integral nonlinearity (INL) was calculated from the dynamic DNL of the digitiser. Then, the measured values of code bin width (CBW) of the digitisers were derived from their DNL and used for the digitisation of ideal lightning impulse waveforms. The lightning impulse parameters of the ideally digitised waveforms and those of the waveforms digitised with the measured CBW were compared. The results showed that the parameter errors caused by the dynamic non-linearity of the digitisers were more affected by the shape of their INL curves, rather than the DNL. The differences in the errors of front time (T-1) and time to half-value (T-2) caused by the two types of test voltage for obtaining the CBW were within 0.9% and 0.6%, respectively. The method used in this study may be used for estimating the uncertainties of lightning impulse measurement caused by dynamic non-linearity of the digitiser.