[SI-LIST] Re: [SI-List]: Current Vs Voltage transmission for better SI

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 02 May 2014 00:45:46 -0700

A current mode driver can be designed to operate in Class A only.  Such 
a driver only has to pull down (or up) by a varying degree to represent 
the drive level desired at a given moment.  A voltage mode driver has to 
pull up and/or down to each output level.  Multiple transistors must 
turn and off to effect a change from one output drive level to another.

 From an SI standpoint it is easier for a chip designer to get a high 
degree of symmetry between the two lines of the pair using current 
mode.  As a board designer you don't care how hard or easy the chip 
design is.  You care about what the chip delivers at the package / PCB 
interface.

Steve

On 5/2/2014 12:14 AM, vinod ah wrote:
> Hi All,
> I have a basic query on usage of current waveform or voltage waveform for
> transmission of signals from Serdes driver to receiver over PCB channel.
>
> I understand that from a chip driver design point of view, current output
> is preferred.
>
> But i am trying to understand what benefits do we get from Signal integrity
> (from board design aspect we do get benefit of no did/dt issues) point of
> view if we transmit a current waveform over the channel (which ultimately
> gets converted to voltage at Rx termination) compared to voltage waveform.
>
> I did read some articles on web which say current waveforms are more
> resistant to interference/noise, not sure why is it so.......
>
> Regards
> Vinod A H
>
>
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