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Design >> Mixed-Signal Design >> [Query] Current Steering DAC
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Message started by raduga on Feb 15^th, 2007, 4:28am

Title: [Query] Current Steering DAC
Post by raduga on Feb 15^th, 2007, 4:28am

Hi all,

I have started a design of a high speed Current steering DAC.
The design is basically a fully differential design. The primary
question which I am facing is about the choosing the values
of the load Resistor RL. Please let me know if there is any standard
way of choosing this value; as most of the designs which I have seen
in literature has been 25/ 50 / 75 ohm but they haven't dwell on justifying
their choice of the Load Resistor's value.

Thanks in advance.

Regards

Raduga

Title: Re: [Query] Current Steering DAC
Post by panditabupesh on Feb 15^th, 2007, 10:37am

These resistance values seem to be dictated by termination considerations.

Bupesh

Title: Re: [Query] Current Steering DAC
Post by Visjnoe on Feb 15^th, 2007, 10:45am

Dear Raduga,5

the load resistance is related to the termination resistance required for your application and as such, is not
a design variable.

E.g. in video applications the typical load resistance would be 75 Ohm.

Kind Regards

Peter

Title: Re: [Query] Current Steering DAC
Post by SRF Tech on Feb 15^th, 2007, 11:50am

Raduga5,
Peter is right, the resistance is dictated, generally by the application. More specifically, your load resistance must match the impedance of the transmission line you are meant to drive, and terminate it (as Bupesh implied).
As Peter noted, video applications use 75ohms T-Lines (the type of coax or RCA cable that conveys the signals), most backplanes use 50 ohms trace routes, some chip to chip interconnect use 65 ohms (easy to design in a package). If you do not impedance match the T-line line of your application, welcome to a signal integrity nightmare, and likely a failed design.
-Stephen

Title: Re: [Query] Current Steering DAC
Post by raduga on Feb 22^nd, 2007, 2:47am

Thanks Bupesh, Peter and Stephen .. !!

As you guys have said that the termination dominates this selection, what could be the case
if it would be like this DAC is feeding it's output to a MOS modulator ? In that case
the DAC would be looking at the input impedence of a MOSFET and effectively the
resistance it would look at would be infinite and some gate capacitace would shunt
the resisances .. so in this case how should we select the resistance value ?

Thanks in advance ..

Regards
Raduga

Title: Re: [Query] Current Steering DAC
Post by vivkr on Feb 22^nd, 2007, 2:56am

Hi Raduga,

Is this load device (MOS) on-chip? If so, then you may not have to worry about the termination
issues and can choose any convenient value, dictated by other design constraints (speed,power etc.).

Termination is generally required when interfacing to and from the external world. Tom Lee covers
the topic in his book on CMOS RF IC Design in the chapter on "The Smith Chart and s-parameters".

Regards
Vivek

Title: Re: [Query] Current Steering DAC
Post by panditabupesh on Feb 22^nd, 2007, 8:08am

Hello
DAC feeding its output to a MOS modulator- do you mean it is an input to the integrator of the modulator. If yes then why do you want to select any resistance (load ) for the DAC?

Bupesh

Title: Re: [Query] Current Steering DAC
Post by raduga on Feb 22^nd, 2007, 10:18pm

Thanks Vivek and Bupesh ..

Bupesh, the modulator is a RF modulator module .. and there could be a filter before the modulator
.. and i just know that the input it gonna see is the gate of a MOSFET .. so thats the problem ..
the typical block diagram of the system is like given below .

Title: Re: [Query] Current Steering DAC
Post by panditabupesh on Feb 23^rd, 2007, 8:14am

Hello
I think there are two issues I would consider while deciding the resistance value:

1. the un-modulated data rate. The Cin of next stage (Cdb etc also included), filter or mixer, together with Rload create an output pole.
2. the swing desired at the output of the DAC.

Based upon 1. and 2. a buffer, source-follower for example, maybe needed between the DAC output and the next stage.
For a slightly different application I once used a transimpedance amplifier with a R parallel C as the feedback impedance.

Bupesh