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Design >> Mixed-Signal Design >> Understanding DAC SFDR
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Message started by analog2001 on Nov 7^th, 2011, 7:14pm

Title: Understanding DAC SFDR
Post by analog2001 on Nov 7^th, 2011, 7:14pm

Hello,
I have a basic question about DAC SFDR.

I modeled an ideal 16bit dac in verilog-a, I apply an ideal sine wave of 400MHz to an ideal 16bit adc, the adc drives the dac. When I use a fourier transform of the output of the dac, the clock rate is 8GHz, I get -92dB THD. However, if I drop the clock rate to 1.6GHz the THD drops to -6.9dB. I am plotting the 1st 16 harmonics.

When I look at ADI, Maxim, NXP etc their input data rate is 1/4 of the clock rate but their dac( which are not ideal) are showing much better results

any help will be greatly appreciated
thanks analog2001

Title: Re: Understanding DAC SFDR
Post by loose-electron on Nov 8^th, 2011, 4:13am

SFDR and THD are two different things.

Title: Re: Understanding DAC SFDR
Post by raja.cedt on Nov 8^th, 2011, 7:31am

hello analog2001,
What's your Question, is it about SFDR or THD? SFDR is about largest harmonic and THD is about all harmonics

Any how it's very common to observe SFDR drop at higher sampling rate due to several frequency dependent degradation like o/p impedance of the tail current source, delay variation to current steering switch. In general people are intersted SFRD at frequencies near to nyquist rate.

Thanks,
Raj

Title: Re: Understanding DAC SFDR
Post by sheldon on Nov 10^th, 2011, 5:07am

Analog2001,

First, what you are saying really does not make sense. Since you are
using an ideal ADC, DAC the SNDR will be ~96dB or there is a problem
with your models. In addition, an ideal ADC, DAC will not have distortion
so why are you looking for it? An ideal ADC, DAC will only have quantization
noise. So my suspicion is that there is a testbench or a measurement
issue. More details about the testbench and measurement would help
to clarify the source of the issue.

Sheldon

Title: Re: Understanding DAC SFDR
Post by PW on Feb 26^th, 2012, 12:11pm

Hi Analog2001,
The SFDR is the difference in dB between the fundamental and the higher harmonic. Thus, the SFDR is generaly higher than the THD.

Title: Re: Understanding DAC SFDR
Post by BackerShu on Jul 18^th, 2012, 10:50pm

Clear answer has been given by raja. Attaching this just in case who check this post later and want to learn more. A very good document on basic definition of ADC from NS.

http://robosavvy.com/Builders/limor.bak/ABCs_of_ADCs.pdf