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4.2

Coherent Input Frequency Selection

ADC Evaluation

Typical ADC analysis requires users to collect the resulting time-domain data and perform a Fourier

transform to analyze the data in the frequency domain. A stipulation of the Fourier transform is that the

signal must be continuous-time; however, this is impractical when looking at a finite set of ADC samples,

usually collected from a logic analyzer. Consequently, users typically apply a window function to minimize

the time-domain discontinuities that arise when analyzing a finite set of samples. For ADC analysis,

window functions have their own frequency signatures or lobes that distort both SNR and SFDR

measurements of the ADC.

TI uses the concept of coherent sampling to work around the use of a window function. The central

premise of coherent sampling entails that the input signal into the ADC is carefully chosen such that when

a continuous-time signal is reconstructed from a finite sample set, no time-domain discontinuities exist. To

users select the frequency of interest for evaluating the ADC; then, they divide this by the fundamental

frequency. This typically yields a non-integer value, which should be rounded to the nearest odd,

this with the fundamental frequency to obtain a coherent frequency to program into their ADC input signal

generator. The procedure is summarized as follows.

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SLAU205 – January 2008

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