Hector Martin wrote:
> I'm working on my first project involving video-frequency analog 
> signals, with everything that they involve (termination, AC coupling, 
> layout considerations, et al). I'm looking for HOWTOs or guides on the 
> topic, geared towards someone who has worked with digital electronics 
> and simple low-frequency analog systems. Though I think I sort of get 
> how it works, I still have many doubts about the proper ways of 
> interconnecting the different components and things such as 
> back-termination and proper DC biasing and AC coupling.

First off, "video-frequency" ain't what it used to be.  It 'used' to be 
about 5MHz; you're working with something faster; note those 1.5nS rise 
times on the data sheet and the 400MHz bandwidth.


> I'm particularly confused by the input connections to the LMH1251. 
> Currently I've just copied the schematic on Page 8 of the datasheet, but 
> I thought the 75ohm resistor should be *across* the inputs (center to 
> ground, as a termination), not in series with ground. The latter seems 
> especially pointless, since the RCA jacks will likely end up bridged to 
> ground through the metal case anyway. 

Ignore that.  It's either a complete mistake or it's for a special test 
fixture at National.  Note figures 1 & 2 --- none of that stuff is shown 
there; only in figure 3 labeled "Test Circuit"

 >I'm also not entirely sure about
> the values to use for the AC coupling capacitors either. I'm thinking I 
> should probably reverse the polarity of the electrolytic capacitors as 
> well, as the right side is biased to 1/2Vcc and thus should be at a 
> higher potential than the left side.

If the biasing is as you describe, you are right.  In any case, the caps 
should not be reversed biased.

> I'd appreciate it if someone had a look at the schematic to point out 
> any places I may have gone wrong. I'm also looking for any information 
> on how I should go about laying this down on a PCB. Should I use a 
> ground plane behind everything? 

I would.

>What about the width of traces? 

As the data sheet says: short.

>Am I fine as long as I keep the traces short enough? How short is "short 
> enough"?

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?

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.

Good luck!


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