The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Mixed-Signal Design >> Loading Consideration for Current Steering DAC
https://designers-guide.org/forum/YaBB.pl?num=1210748326

Message started by raduga on May 13^th, 2008, 11:58pm

Title: Loading Consideration for Current Steering DAC
Post by raduga on May 13^th, 2008, 11:58pm

Hi All,

I am wondering if any one of you can tell me the reasoning behind choosing the output resistances for any current steering DACs
as 25, 50, 75 and 100 ohm. I mean choosing such a small value of resistance we have to sacrifice a lot of power.

TIA

Raduga

Title: Re: Loading Consideration for Current Steering DAC
Post by loose-electron on May 14^th, 2008, 3:55pm

50 ohm and 75 ohm coax being driven by a DAC often uses the resistors in the DAC as the reflection termination resistors.

Also -- Most current steering DAC's are for high frequency applications and the need os for lower impedances to not allow parasitics to become a problem.

Title: Re: Loading Consideration for Current Steering DAC
Post by Monkeybad on May 14^th, 2008, 8:13pm

Hi, raduga!
Besides the high speed consideration, linearity is another reason for using low output resistances.
The output impedance and the parasitic impedance of an non ideal current source will affect the linearity.
Define Rratio=Ro/Rload, where Ro is the parasitic impedance of an non ideal current source looking from the Drain and Rload is the output loading resistance.
The higher the Rratio is, the better the linearity of a DAC is.
You can refer to "Cmos Data Converters For Communications, Gustuavsson, Wikner and Tan"
In Chapter 12 there are some discussions about the output resistances effect.
Hope this will provide some help.

Title: Re: Loading Consideration for Current Steering DAC
Post by raduga on May 14^th, 2008, 9:29pm

Dear loose-electron and Monkeybad,

Thanks a million for your help.. Now I got the point crystal clear !!

Regards

Raduga