A DNL measurement on an oversampling noise shaping ADC

Oversampling Noise Shaping ADCs (ONSAs) are recognised as being capable of very good low signal performance, because they do not exhibit the Differential Non Linearity (DNL) and variations in Differential Gain that other ADC architectures exhibit. DNL generally causes subsequent signal processing to break down as the signal gets smaller, and is a very common cause of problems with ADCs. Although ONSAs are expected to have very good (low) DNL, there have, to our knowledge, been no reported measurements made on the DNL to any great accuracy. We therefore set out to measure the DNL of an ONSA, using the histogram technique, to see how low it was. We used a multibit third order ONSA, sold for audio applications, that uses 128* oversampling (dCS 900B). The result was that the DNL was extremely low, and so we developed a theory to estimate the rms DNL of the converter, and separate it from the (Bernoulli sampling) noise on the measurement. This paper reports the theory, and the results of the measurement