Measuring Wideband Nonlinearity of Analog to Digital Converters

Measuring the nonlinearity of an analog to digital converter (ADC) is often limited by the nonlinearity of the test signal. A differential test signal that combines two comb spectra has been used to perform nonlinearity measurements that are not limited by the nonlinearity in the digital to analog converter (DAC) used to generate these comb signals. Each of the differential inputs of the ADC is connected to one of the comb signals with different frequencies and the ADC effectively sums both spectra internally. This ensures that any intermodulation products in the ADC output originate in the ADC itself. Harmonic related products of the comb spectra are generated by the two DACs and the ADC. The out of band spectrum shows the combined effect of both DAC nonlinearity and ADC non-linearity and includes both harmonic and intermodulation products. Although the harmonics of each test signal are hidden in the comb spectra, the hidden harmonic distortion power has been shown to be orders of magnitude below the intermodulation power. An FFT plot of the ADC output in principle enables a noise power ratio (NPR) measurement to be made but this would be limited by waveform generator nonlinearity to less than 18 bits. Comb filtering removes the harmonic spectra and harmonic products from the output of the ADC. The intermodulation products, in band and also out of band, are thus revealed. In contrast to NPR measurements, the measurement of wideband ADC ENOB is not limited by the nonlinearity in the signal generator. We present the characterization of an audio test measuring system /audio analyzer using this technique and sine waves. The nonlinearity of the DAC was measured as 18 effective number of bits (ENOB) and the internal ADC as 22 ENOB. The validated dual comb test waveform is then used to measure the nonlinearity of a 24 bit 2 M samples per second ADC.