03-25-2019 07:51 AM
When using LVDS inputs it is clear that there is a 100 Ohm differential termination inside the FPGA that can be enabled or disabled as needed. My question is whether there is a termination built into the LVDS output driver. I understand that there are no options to configure this, so it must be something intrinsic to the driver.
The reason we are asking is because we are trying to understand how the current out of the LVDS driver changes depending on the value of the resistor placed across the P/N pair. We tried different values of resistor from 50 Ohm to 1k Ohm, and the current goes down the higher the value of resistor. This would seem to imply that the remainder of the current gets diverted through an internal resistance between P and N if the LVDS driver indeed provides a constant total current. Also, if no resistor is placed across the P/N pair (open circuit), there must be some kind of internal path for the current to flow between them.... that would also imply that there is a resistive termination inside the driver.
We have also exported the IBIS model to try to understand how the driver works through the Voltage/Current ratio, but it's hard to arrive at an answer to this question through this method.
Any help would be greatly appreciated.
03-25-2019 07:59 AM
No internal termination on the drivers,
Rember LVDS is a current source system, not voltage, OK, real drivers are multiple transostros circuits, current mirrors etc,
Also , of interest, there are no real resistors on the receivers,
they are also current mirrors, ( real termination resistors are to BIG at the silicon scale )
03-25-2019 08:45 AM
03-25-2019 10:24 AM
yes, in the perfect world of theory your right,
no termination, and the output voltgae would rise to infinity trying to drive the current !!
Have a think how you would make an output dirver circuit in the fpga using transistors ?
The output drivers have a compliant range they can drive over, look in the data sheets,
This is all very esoterical sort fo good coffee room chat,
but can I ask what your trying to achieve here ?