ADC Standard IEC 60748-4-3: Precision Measurement of Alternative ENOB Without a Sine Wave

A practical analog-to-digital converter (ADC) introduces quantization error in excess of the ideal value and one way of expressing this is by comparing the value of this error with that of an ideal ADC. This comparison is known as the effective number of bits (ENOBs). It is accepted practice to measure ENOB using the signal-to-noise and distortion (SINAD) ratio of a sine-wave input. This paper extends ENOB theory to any arbitrary waveform by including the crest factor of the input signal. It is now possible to apply the ENOB concept to wideband systems. Measuring the SINAD of an arbitrary or multitone waveform with precision normally requires the use of laboratory standard test equipment. However, International Electrotechnical Commission standard 60748-4-3 specifies an alternative method for wideband SINAD measurements that may also be suitable for built-in test. It is essentially a multitone test using two pseudorandom signal sources and is sometimes known as the double comb-filter (DCF) method. This paper demonstrates the requirements for a practical implementation of a DCF-based system for measuring an ENOB of up to 24 bits. It is shown that in a practical application, DCF ENOB and sine-wave ENOB results have similar levels of accuracy, but in the presence of amplitude nonlinearity the differing test signal amplitude weightings cannot fundamentally produce the same ENOB figure. It is shown that DCF ENOB is more representative of communications system performance and therefore extends the use of ENOB to wideband applications.