选择最佳测试音和测试设备的成功高速ADC正弦波

• Application Note: Coherent Sampling Calculator (CSC)
• Coherent Sampling Calculator (XLS, 81K)

1. Start with fIN = 17MHz and fSAMPLE = 120MHz to determine the window size NWINDOW (remember that according to the previous discussion, NWINDOW has to be an integer odd or mutually prime number) for an 8192-point data record NRECORD.
   
   NWINDOW = int (fIN / fSAMPLE) × NRECORD
   NWINDOW = int (17MHz / 120MHz) × 8192 = 1160
2. Based on the above result for NWINDOW, the next closest mutually prime (odd) number is 1163 (1161). Use either of these numbers to compute the final, near-optimum input test tone as follows
   
   fIN = fSAMPLE × (NWINDOW / NRECORD)
   fIN(MUTUALLY_PRIME) = 120MHz × (1163 / 8192) = 17.0361328MHz
   fIN(ODD) = 120MHz × (1161 / 8192) = 17.0068359MHz

1. Determine the resolution of the sampling frequency that fits into an 8192-point record by
   
   Δf = fSAMPLE / NRECORD
   Δf = 120MHz / 8192 = 14.6484375kHz
   
   
2. Some of commonly available signal generators in the market may not offer enough resolution to offer this many digits to accurately capture both input and sampling frequency. To bypass this requirement and still meet the coherent sampling condition, it is recommended to select Δf based on the next highest integer number.
   
   Δf = int (fSAMPLE / NRECORD) = 15kHz
   
   
3. Based on the new Δf, the exact sampling frequency computes to
   
   fSAMPLE = Δf × NRECORD
   fSAMPLE = 15kHz × 8192 = 122.880MHz
   
   
4. Δf also helps to determine the size of NWINDOW. Again, use the next highest integer odd (or mutually prime) number, determined by the desired input test tone and Δf.
   
   NWINDOW = int (fIN / Δf)
   NWINDOW = 17MHz / 15kHz = 1133
   
   
5. Based on these findings, the near optimum input test tone fIN calculates as follows
   
   fIN = fSAMPLE × (NWINDOW / NRECORD)
   fIN = 122.88MHz (1133 / 8192) = 16.995MHz

Equipment and Set-Up Recommendations for a Successful High-Speed ADC Test

%Source Code Sample MAX144X Family
%The following program code plots the FFT spectrum of a desired test tone. Test tone based on coherent sampling criteria, and
%computes SNR, SINAD, THD and SFDR.
%Copyright Au/Hofner, Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA94086
%This program is believed to be accurate and reliable. This program may get altered without prior notification.;
disp('HP16500C LA 100/110 State Card');
filename=input('Enter file name or press RETURN to accept data from LA via GPIB/HPIB): ');
if isempty(filename)
filename = 'listing';
end
fid=fopen(filename,'r');
numpt=input('Number of Points in FFT? ');
fclk=input('Sampling Frequency (MHz)? ');
numbit=input('ADC Resolution? '); 
 
%Discard first 13 lines of the LA listing (LA header), as they don't contain valid data.
for i=1:13,
fgetl(fid);
end 
[v1,count]=fscanf(fid,'%f',[2,numpt]);
fclose(fid);
v1=v1'; 
code=v1(:,2);
%Warning: ADC output may be clipping - reduce input amplitude
if (max(code)==2numbit-1) | (min(code)==0)
disp('WARNING: ADC OUTPUT MAYBE CLIPPING - CHECK INPUT AMPLITUDE!');
end
figure;
plot([1:numpt],code);
title('TIME DOMAIN')
xlabel('SAMPLES');
ylabel('DIGITAL OUTPUT CODE');
Dout=code-(2^numbit-1)/2;
%Re-center the digitized sinusoidal input
Doutw=Dout;
Dout_spect=fft(Doutw);
Dout_dB=20×log10(abs(Dout_spect));
figure;
maxdB=max(Dout_dB(1:numpt/2));
plot([0:numpt/2-1].×fclk/numpt,Dout_dB(1:numpt/2)-maxdB);
grid on;
title('FFT PLOT');
xlabel('ANALOG INPUT FREQUENCY (MHz)');
ylabel('AMPLITUDE (dB)');
a1=axis; axis([a1(1) a1(2) -100 a1(4)]);
fin=find(Dout_dB(1:numpt/2)==maxdB);
%Find the signal bin (DC represents bin=1)
span=max(round(numpt/200),5); 
%Determine span of input frequency on each side
spanh=2; 
%Search span for harmonic distortion components on each side
spectP=(abs(Dout_spect)).× (abs(Dout_spect));
%Determine power level
Pdc=sum(spectP(1:span));
%Determine DC offset power level
Ps=sum(spectP(fin-span:fin+span));
%Determine signal power level
Fh=[]; 
%Vector storing frequency and power components of signal and harmonics
Ph=[]; 
%HD1=signal, HD2=2nd harmonic, HD3=3rd harmonic, etc.
%Find the harmonic frequencies/power within the FFT plot
for har_num=1:10
tone=rem((har_num × (fin-1)+1)/numpt,1); 
%Note: tones > fSAMPLE are aliased back
if tone>0.5
tone=1-tone; 
end 
Fh=[Fh tone]; 
%For this method to work properly, make sure that the folded back high order harmonics do not overlap with DC and signal
%components or lower order harmonics.
har_peak=max(spectP(round(tone×numpt)-spanh:round(tone×numpt)+spanh));
har_bin=find(spectP(round(tone×numpt)-spanh:round(tone×numpt)+spanh)==har_peak);
har_bin=har_bin+round(tone×numpt)-spanh-1; Ph=[Ph sum(spectP(har_bin-1:har_bin+1))];
end
Pd=sum(Ph(2:5));
%Total distortion power level
Pn=sum(spectP(1:numpt/2))-Pdc-Ps-Pd;
%Extract noise power level
format;
A=(max(code)-min(code))/2numbit
%Analog input amplitude in mV
AdB=20×log10(A)
%Analog input amplitude in dB
SNR=10×log10(Ps/Pn)
%SNR in dB
SNR=10×log10(Ps/Pn)
%SINAD in dB
SINAD=10×log10(Ps/(Pn+Pd))
disp('THD - HD2 through HD5');
THD=10×log10(Pd/Ph(1))
%THD in dB
SFDR=10×log10(Ph(1)/max(Ph(2:10)))
%SFDR in dB
disp('SIGNAL AND HARMONIC POWER (dB)');
HD=10×log10(Ph(1:10)/Ph(1))
hold on;
plot(Fh(2)×fclk,0,'mo',Fh(3)×fclk,0,'cx',Fh(4)×fclk,0,'r+',Fh(5)×fclk,0,'g×',Fh(6)×fclk,0,'bs',Fh(7)×fclk,0,'bd',Fh(8)×fclk,0,'kv',
Fh(9)×fclk,0,'y^');
legend('SIGNAL','HD2','HD3','HD4','HD5','HD6','HD7','HD8','HD9');
hold off;

Conclusion

1. Coherent Sampling vs. Window Sampling
   
2. Defining and Testing Dynamic Parameters in High-Speed ADCs, Part 1
   
3. Dynamic Testing of High-Speed ADCs, Part 2
   

1. If NRECORD is a power-of-two value, then an odd number for NWINDOW will meet the coherent sampling requirement.
   
2. The size of an ideal frequency bin is defined by bin = fSAMPLE / NRECORD.