SRF Tech wrote on Nov 10th, 2006, 6:29am:Dandelion,
You have the current source placed correctly (though I believe you meant to label your output buffer a transmitter and not a reciever) and if you are doing simulations to validate specs (Voh, etc), I would stick with the current source as a resistor will be impacted by the dynamic output resistance of your transmitter, making it difficult to control Io for spec validation purposes.
If what you are doing, assuming I understand correctly, is validating your spces (Voh, Vol, Io, etc), the only condition with the current source is to get the polarity right. For Vol, you are sourcing current into the buffer so your polarity as you have drawn it would be positive (+Io), while for Voh you need to be sinking current out of the buffer so your polarity should be negative (-Io). What you are testing it for, in the Voh example, is that even when the part is sourcing Io to the load, it is still maintaining a voltage high enough to meet Voh specs, and vice versa for Vol. (note I am assuming |Iol| = |Ioh|, which may not be true in which case make sure to match the polarity with proper value +Iol and -Ioh).
Note that this approach is how you would consider simulations for TTL or ECL type circuits. Sometimes, in say a GTL logic family, certain aspects of these specs make no sense, such as in GTL, you would never expect to see a -Io in your schematic as all loads are pull up and so assuming the drivers voltage is never higher than the pull-up Vcc, Io will always be positive. (polarities are specific to your schematic). However, you can still simulate for the specs.
I am not quite sure I understand your reciever question. Recievers do not have Voh and Vol specs per se, but they do have to trigger appropriately for such values as Vil and Vih, and how you simulate these voltages depends on your IO standard.
I hope this helps,
SRF
Hi SRF,
Thanks for the reply. And I also apologizes not demonstrate my question clearly.
Pls. see the attached files, It is the datasheet of the LVDS receiver. In the specifications, the yellow part gives the Voh and Vol spec. We are designing one circuit like this in our project.. According the test condition in the spec, I drafted the above diagram.
When I validating the Voh and Vol spec using the above digarms for all the PVT corners. I found the Voh is always near at 5V and Vol is always near 0 for all the PVT corners. I wonder why the much difference compared with the spec, after all, it is low to 3.8V in Min for Voh and high to 0.3V in MAX for Vol
The question here is that we used the ideal current source. so we can not get the correct output level like the actual load, Some one suggested me to use the resistor to replace the ideal current source, i.e., select the resistor properly to get the needed current and monitor the output, then the results is near the spec now.
Another questions, in actual test, we indeed need to use an current source to test it? We have no current source in our lab, so we wonder if we can use the resistor also.
P.S. We need not worry the dynamic output resistance, because the test is static and no switching occurred.
Pls. comment.
Thanks a lot.
Dandelion