[SI-LIST] Re: 50ohm SSTL2 Tx

Rajat, I can't answer your question directly.  It's all in the IV 
curves.  I would pose some gating questions which will help you decide.

Can your driver source 8ma at Vout of 1.65 volts?  An SSTL2 class 1 
driver can.

Can it sink 8 ma at Vout of 0.85 volts?  An SSTL2 class 1 driver can.

If not, you have built a 50 ohm 2.5 volt CMOS driver. 

It may be compatible with an SSTL2 input - it may not.  The SSTL specs 
are written so that an incident wave will switch the receiver.  If you 
are driving a point-to-point net, you can do it with your driver.  If 
there's a load in the middle of the net, it will see zero or nearly so 
input differential until the reflection comes back from the open end of 
the line.  What will it do when presented such an input?


SSTL2 class 1 is defined to drive a 50 ohm load terminated to VDD/2, 
which produces approximately the operating points described above.  For 
a class2 driver, the currents are 16 ma, and the load impedance is 25 ohms.

There may be new classes, for driving unterminated lines, added to the 
spec into which your driver may fall.

Regards

Mike



Rajat Chauhan wrote:

> Hi all, 
>  I am asked to design SSTL2 Tx to drive 50ohm line (unterminated). What
>i did is, sized my buffer to give first step equal to VDDS/2 at near end
>with 50ohm line connected. Without giving any look to SSTL2 JEDEC specs.
>It can drive SSTL2 Rx.
>   The confusion is, is this realy a SSTL2 Tx?
>   If Yes, how do i label it as class-I or class-II, because it comes
>out to be some where in between the two? For class-II it is weak and for
>class-I it is too strong.
>   If No, than that means we cannot have good SSTL2 Tx to drive 50ohm
>line (unterminated)?
>
>I am stucked....please help, i am missing something.
>Thanking in advance,
>Regards
>Rajat
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