AD7569 datasheet(13/20 Pages) AD

13 / 20 page

AD7569/AD7669

–13–

REV. B

Figure 15. DAC Output Spectrum

HISTOGRAM PLOT

When a sine wave of specified frequency is applied to the VIN in-

put of the AD7569/AD7669 and several thousand samples are

taken, it is possible to plot a histogram showing the frequency of

occurrence of each of the 256 ADC codes. If a particular step is

wider than the ideal 1 LSB width, the code associated with that

step will accumulate more counts than for the code for an ideal

step. Likewise, a step narrower than ideal width will have fewer

counts. Missing codes are easily seen because a missing code

means zero counts for a particular code. The absence of large

spikes in the plot indicates small differential nonlinearity.

Figure 16 shows a histogram plot for the ADC indicating very

small differential nonlinearity and no missing codes for an input

frequency of 204 kHz. For a sine-wave input, a perfect ADC

would produce a cusp probability density function described by

the equation

p(V )

π( A2 −V 2)1/2

where A is the peak amplitude of the sine wave and p(V) the

probability of occurrence at a voltage V.

The histogram plot of Figure 16 corresponds very well with this

cusp shape.

Further typical plots of the performance of the AD7569/AD7669

are shown in the Typical Performance Graphs section of the data

sheet.

Figure 16. ADC Histogram Plot

and harmonic distortion performance. Similarly, for inter-

modulation distortion, an input (either to VIN or DAC code)

consisting of pure sine waves at two frequencies is applied to the

AD7569/AD7669.

Figure 13. ADC FFT Plot

Figure 13 shows a 2048 point FFT plot of the ADC with an in-

put signal of 130 kHz. The SNR is 48.4 dB. It can be seen that

most of the harmonics are buried in the noise floor. It should be

noted that the harmonics are taken into account when calculat-

ing the SNR. The relationship between SNR and resolution (N)

is expressed by the following equation:

SNR = (6.02N + 1.76) dB

This is for an ideal part with no differential or integral linearity

errors. These errors will cause a degradation in SNR. By work-

ing backward from the above equation, it is possible to get a

measure of ADC performance expressed in effective number of

bits (N). This effective number of bits is plotted versus fre-

quency in Figure 14. The effective number of bits typically falls

between 7.7 and 7.8, corresponding to SNR figures of 48.1 dB

and 48.7 dB.

Figure 15 shows a spectrum analyzer plot of the output spec-

trum from the DAC with an ideal sine-wave table loaded to the

data inputs of the DAC. In this case, the SNR is 46 dB.

Figure 14. Effective Number of Bits vs. Frequency

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