Abstract
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.
Highlights
A PURE sine wave or tone is commonly used for testing the linearity and resolution of Analog-to-digital converter (ADC) circuits as it can be made close to ideal using suitable filtering and has the advantage that harmonic measurement equipment is available commercially to check the purity of the source
This paper has shown that the double comb-filter (DCF) wideband multitone method specified in International Electrotechnical Commission (IEC) standard 60748-4-3 can be better defined so that appropriate choices of test signal frequencies and measurement filter can be made
The signalto-noise and distortion (SINAD) or signal to quantizing noise ratio (SQNR) of an ideal ADC can be measured with the DCF system within 0.1 dB of the SINAD or SQNR of an ideal sine wave
Summary
A PURE sine wave or tone is commonly used for testing the linearity and resolution of Analog-to-digital converter (ADC) circuits as it can be made close to ideal using suitable filtering and has the advantage that harmonic measurement equipment is available commercially to check the purity of the source. In 2006, the publication of the International Electrotechnical Commission (IEC) standard IEC 60748-4-3 [1] provided for wideband measurement of ENOB with an alternative test signal. It includes input from IEEE [2] and the national standards bodies of IEC member countries worldwide. This section of the standard specifies results that relate only to the wideband noise-like test signal At the time this standard was written, there was no evidence to indicate that measurements made with this DCF test system could be directly related to the ENOBs of a sine wave, and for that reason the test was intended for routine rather than type-approval use
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