Doubt in THD in ADC and DAC.....

Hi!

i have doubt regarding the THD of the ADC and DAC ,....first i will explain what my understanding was...

in case of DAC THD means it is RMS ratio of sum of harmonics(generally 6) power to actual signal to be generated....

for ex THD = -60db means all harmonics power sum will be some thing like 1/10^6 times of power of actual signal...

whether it is correct or any thing to be corrected....

how can we have to understand this regarding ADC ....because for DAC output will be analog which will contain actual signal+harmonics+some times image frequencies+spurs , so, we can interpret the THD easily in case of a DAC....\\\
But for ADC output is digital , how can we interpret harmonics,spurs etc in the digital output......
i am weak in basics....if i am wrong please correct me....if what i am telling is not understandable,please let me know....

if anybody can answer it will be very helpful to me..

thanking u,

waiting for reply.....

You need to reconstruct the ADC output and then run a DFT on it (the equivalent of a spectrum analyzer for an analog signal). So, if you have a 4-bit ADC:
y=16*d[3]+8*d[2]+4*d[1]+d[0]; y is your reconstructed sinewave, d[4:0] is your ADC output.
Once you calculate DFT, you can add the harmonics in power and compare to the input signal power.

Thanks for the reply....

what u r telling is if an ADC has sampled 1cycle of sine wave to some 1024 samples ....then for each digital value we have to take the expansion and convert it into a sample value....and then applying the DFT to all the Discrete data......and see the harmonics in the spectrum......

Thank you....i am understanding it..but, if i see it i will get more IDEA regarding this ....
can we have any tools so, that i will take samples from ADC and then to see the spectrum of the Output....

like in Multisim or Hspice can we see or interpret it......

thanking u,

If you want to see how to use the .FFT statement in HSPICE check here.
You will need to write the formula '16*v(d3)+8*v(d2)+4*v(d1)+v(d0)' (including the quotes) in the <output_var> field, where d3...d0 are the output nodes of the ADC.

Ok thank u....

i will try and get back friend////

i have a similar problem..

now, i'm designing 5b sar adc.
for the analog part (like comparator, switch, caps), i'm using HSPICE. for the digital part (sar logic), i'm using verilog-HDL.

to simulate it, i wrapped the analog part to verilog, and then simulated them using nanosim. (it is an AMS simulation).

to see the waveform, i use cscope.firstly, i converted the digital output to analog using d2a tools that is provided by cscope, and then using cscope calculator, i did fft analysis of that signals.

the problem is:
1.how to measure the SNR?
2. is my way correct?

thanks for the reply

Remember you can take the ADC output into an ideal DAC, get the sinewave and run THD on that.