# Speed difference between ADCs and DAC's

**URL:** <https://www.ruby-forum.com/t/speed-difference-between-adcs-and-dacs/219965>\
**Category:** GNU Radio\
**Created:** [May 23, 2012, 11:43am UTC](https://www.ruby-forum.com/t/speed-difference-between-adcs-and-dacs/219965 "2012-05-23T11:43:10Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![dubstep](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/cdc98d/32.png) [@dubstep](https://www.ruby-forum.com/u/dubstep)\
**Post date:** [May 23, 2012, 11:43am UTC](https://www.ruby-forum.com/t/speed-difference-between-adcs-and-dacs/219965/1 "2012-05-23T11:43:10Z")

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hi everyone,

USRP board has ADC’s and DAC’s operating with different speeds. why  
DAC’s  
are faster than ADC’s? please excuse me if it is a basic question.

thanks in advance,  
sravya.

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [May 23, 2012, 5:42pm UTC](https://www.ruby-forum.com/t/speed-difference-between-adcs-and-dacs/219965/2 "2012-05-23T17:42:38Z")

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ADI tutorial on the topic.  
[http://www.analog.com/static/imported-files/tutorials/MT-017.pdf](http://www.analog.com/static/imported-files/tutorials/MT-017.pdf)

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [May 27, 2012, 4:31am UTC](https://www.ruby-forum.com/t/speed-difference-between-adcs-and-dacs/219965/3 "2012-05-27T04:31:50Z")

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I dont know if that tutorial covers the topic, try this  
[http://www.analog.com/library/analogDialogue/archives/39-06/Chapter%202%20Sampled%20Data%20Systems%20F.pdf](http://www.analog.com/library/analogDialogue/archives/39-06/Chapter%202%20Sampled%20Data%20Systems%20F.pdf)

I think its simple to say why DAC and ADC operate a different speeds.  
Its not just speed, also dynamic range (ENOB) is also more difficult to  
achieve in ADC.  
Ultimately an ADC must guess at what the analog input is due to internal  
noises (sample and hold hi-z, etc), which is why the ENOB is always  
worse  
than the number of bits output digitally.

By contrast, the DAC since it operates in reverse, it already knows  
exactly  
what the signal should be, and just needs to switch in the correct  
resistor  
to produce the correct analog value.

A high speed ADC/DAC expert would say this is an over-simplification,  
but I  
only try to convey the basic idea.

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**Author:** ![casper\_the\_ghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/58956e/32.png) [@casper\_the\_ghost](https://www.ruby-forum.com/u/casper_the_ghost)\
**Post date:** [May 27, 2012, 8:05am UTC](https://www.ruby-forum.com/t/speed-difference-between-adcs-and-dacs/219965/4 "2012-05-27T08:05:56Z")

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Hi all,

Well going into the basics, ADCs have a sample and hold circuits. It  
holds the value and sometimes takes the average of some nearby values  
and then convert that into digital for precision. While there is no  
such thing in DACs; resulting in faster DACs.

Hope this helps…

Regards,  
Ebtisam