[SI-LIST] Re: [SI-List]: Current Vs Voltage transmission for better SI
- From: Mike <bmgman@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 02 May 2014 11:31:18 -0500
Perhaps an argument can be made that it is easier to get a well-matched
source termination with a current-mode driver. The source impedance,
even with process variations, is relatively high wrsp Zo, so variations
in Zout get swamped by the relatively low, parallel Rt. Contrast
voltage mode drivers where process variation in Zout transfer directly
to the termination impedance
Mike
- - - - -
On 05/02/2014 02:45 AM, steve weir wrote:
> 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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