AD7569/AD7669
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 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 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:
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
SNR = (6.02N + 1.76) dB
1
p(V ) =
π(A2 −V 2 )1/2
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.
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 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
Figure 16. ADC Histogram Plot
REV. B
–13–
ADI [ ADI ]
