My customer has the following question:
Regarding termination of the LMH0307, the datasheet discusses 75 ohm output pull-ups with a 750 ohm RREF resistor. I installed 50 ohm pull-ups to provide a 100 ohm differential output. It looks like the RREF resistor needs to change to 500 ohms as well otherwise I don't get the specified output voltage levels. The datasheet is adamant it should be 750 ohms, however it only addresses 75 ohm terminations. Am I correct the RREF resistor needs to be 10 times the single ended termination resistor value to generate the correct output voltage level? Regards,
Please provide more details about this application. What specific output amplitude is required from the LMH0307, and is this single ended or differential? And what is at the receiving end of the cable?
Typically, there is an SDI adaptive cable equalizer (such as the LMH0344, LMH0384, or LMH0394) at the other end of the cable to receive the signal. For coaxial cable, the single-ended amplitude of the signal going into the cable should be 800 mVP-P in order for the equalizer to properly equalize the signal. For differential cable, the equalizer would be configured to receive a differential signal, and in this case the amplitude of the signal going into the cable should be 800 mVP-P differential (so it would only be 400 mVP-P single ended).
Using 75 ohm pullup resistors, the LMH0307 will drive 800 mVP-P single ended into 75 ohm coaxial cable. With 50 ohm pullup resistors and RREF at the default value of 750 ohms, the single-ended output voltage is about 533 mVP-P ((50/75) * 800). (The differential output voltage is about 1066 mVP-P.) Due to current limitations, the LMH0307 is not able to drive 800 mVP-P single ended into 50 ohm loads. The RREF resistor value can be decreased in order to increase the output amplitude, but the lowest value of RREF that should be used is 665 ohms which provides a single-ended amplitude of about 600 mVP-P into a 50 ohm load.
However, if the intent is to drive a signal into the cable so it can be received differentially by an SDI cable equalizer at the other end, then I would recommend to use the 50 ohm pullup resistors and set RREF to 953 ohms, which will provide 800 mVP-P differential (400 mVP-P single ended) into the 100 ohm differential cable.
Thanks for the information! This is a similar design to the one shown in this forum post (although not identical) http://e2e.ti.com/support/interface/high_speed_interface/f/138/p/199016/716411.aspx#716411
So the output (setup as a 100ohm differential output) from the LMH0307 will be AC coupled to a lightning protection transformer (I've already given them National's paper on lightning protection using other things) through a custom cable, to another transformer and then to the an LMH0344.
It was important to know just how low the Rref could safely be (the older comlinear parts used to give you a reference chart / graph of possible Rref values). I know this is off-dataasheet, but they have been using these for years for various interfaces like this.
Any other recommendations you'd like to offer would be appreciated, although we had a recent conference call with the High Speed Interface guys and the LMH0307 / LMH0344 reference was what they settled on too.
I would recommend using an RREF resistor of 953 ohms for this application. With 50 ohm output pullup resistors, the LMH0307 will drive an 800 mVP-P differential signal into the cable. Assuming there are no other losses in the data path (besides the cable), this is the ideal launch amplitude to allow for the best equalization performance of the LMH0344 equalizer on the other end of the cable.
They should not attempt to drive an 800 mVP-P single-ended (1600 mVP-P differential) signal into the 100 ohm differential cable, for the following reasons:
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