Hector Martin wrote:
> Marcel Duchamp wrote:

> Good. Grounds tied together, 75 ohms across the input then. Are the 
> 33ohm resistors and 220nF caps okay? Should I use a bias circuit like 
> the one on the direct input to the LMH6572, or maybe the schottky diode 
> system they mention in Figure 2? I'm not too sure what DC bias the 
> LMH1251 wants on its inputs (if any).

I don't know the answer to that.  Those two diagrams are for two 
different apps: one's for direct to a monitor while the other goes to a 
buffer that drives a cable.

>>> What about the width of traces? 
>> As the data sheet says: short.
> 
> *width*, not length :)

Oops. Sorry about that.  The traces width should be whatever gives you 
the correct impedance.  Google for trace impedance calculator and you'll 
get plenty of info there.  I guess 75 Ohms is what you're after? I 
haven't the time to go over the datasheet more closely so read it and 
decide.

>> The data sheet says 6pf of loading.  You can calculate this for the 
>> board thickness and dielectric you plan to use.  With smd parts, you 
>> should be able to get them fairly close, no?
> I'm probably going to be using SMD 1206 for the passives. I can do 0805 
> if you think it will help.

Component size should not make a huge difference here. And the 1206 will 
be easier to work with.

>> By the way, did you note those 33Ohm series resistors?  Don't leave them 
>> out.  And if your board shows any ringing on those lines, feel free to 
>> experiment with them by trying both smaller or larger values.  They 
>> absorb ring energy so may need some tweaking to get right.
> 
> I assume you mean the ones (that should be) between the LMH1251 and the 
> LMH6572. I'll add them in.

Yes, definitely add them in.  You can always put zero Ohm jumpers if you 
need to.  Use lots of power supply bypassing as well.  It sounds like 
you are off to a great start with the portion you have prototyped so far.

Tell us how it all works out!












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