Balanced filter -> Balun vs. Balun -> Single-ended filter
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Posted by Joel Koltner on April 1, 2008, 4:59 pm
Please log in for more thread options current-mode output. On the Analog Devices evaluation board schematics, they usually take the outputs through a fully differential LC filter for anti-aliasing and then through a balun to end up with a single-ended output. This of course takes a few more parts than immediately taking the output through a balun and then using a traditional single-ended filter. I've been debating the pros and cons of the two approaches and so far the reasons I can think of for going differential filter and then the balun are... 1) Somewhat more "ideal" filter in that a ground plane (or worse, ground trace) isn't being used for return currents, it's just current flowing from one side to the other of the differential output. (Seems minor, although from past filter designs I've done at UHF this might be a much larger improvement than I'm guesstimating here.) 2) Filtering out the high-frequency scunge before it hits the balun allows the balun to not handle quite as much energy and hence perform a little bit better (more linearly). (Also seems minor.) Am I missing anything else here? Thanks, ---Joel | |||||||||||||
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Posted by John Larkin on April 1, 2008, 7:29 pm
Please log in for more thread options What's the frequency range? We often go into a diff-input opamp first, then a single-ended filter. But then we usually go down to milliHertz frequencies. But in your case, balun (or transformer) then filter seems more sensible. Or if all you want is an RF sine wave, ground one of the outputs and filter the other. No balun! Somebody should make a family of integrated differential-input, single-ended-output amps with integrated, ideally programmable, lowpass filters. They could sell heaps of them, for use with DDS chips and diff-out dacs. The damned filters are 10x the size of the chips! John | |||||||||||||
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Posted by Joel Koltner on April 1, 2008, 8:29 pm
Please log in for more thread options Hi John,
> What's the frequency range?
The output frequency range I care about is 275-300MHz (it's one of their 1GSps DDS chips), so I'm generally avoiding op-amps here. :-) > Or if all you want is an RF sine wave, ground one of the outputs and
> filter the other. No balun! I am just after sine waves. Grounding one output seems reasonable enough, although Analaog Device's advice in this case is to use a doubly-terminated filter on one output (say, 100 ohms at each end) and then terminate the other output with the same effective load (100 ohms double terminated --> 50 ohms).
(This is from
http://www.analog.com/UploadedFiles/Tutorials/450968421DDS_Tutorial_rev12-2-99.pdf , page 47.) > Somebody should make a family of integrated differential-input,
> single-ended-output amps with integrated, ideally programmable, > lowpass filters. They could sell heaps of them, for use with DDS chips > and diff-out dacs. We'd definitely buy a bunch! Thanks for the help, ---Joel | |||||||||||||
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Posted by John Larkin on April 1, 2008, 10:59 pm
Please log in for more thread options On Tue, 1 Apr 2008 17:29:06 -0700, "Joel Koltner"
>Hi John,
> >> What's the frequency range?
>
>The output frequency range I care about is 275-300MHz (it's one of their 1GSps >DDS chips), so I'm generally avoiding op-amps here. :-) > >> Or if all you want is an RF sine wave, ground one of the outputs and
>> filter the other. No balun! >
,
>I am just after sine waves. Grounding one output seems reasonable enough, >although Analaog Device's advice in this case is to use a doubly-terminated >filter on one output (say, 100 ohms at each end) and then terminate the other >output with the same effective load (100 ohms double terminated --> 50 ohms). >(This is from >http://www.analog.com/UploadedFiles/Tutorials/450968421DDS_Tutorial_rev12-2-99.pdf >page 47.)
Gosh, what nonsense. A lot of ADI appnotes are nonsense. We're having interesting problems with their ADUM1400 data coupler driving a floating AD5432 serial DAC. Both have bizarre powerup initilize problems. John | |||||||||||||
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Posted by Joel Koltner on April 2, 2008, 12:41 pm
Please log in for more thread options > Gosh, what nonsense. A lot of ADI appnotes are nonsense.
In this case, is it "this is just unnecessary and doesn't really help so it's a waste of parts"-nonsense or "this is actually harmful compared to the simpler approach"-nonsense? (Or possibly "ADI knows something they're not telling us..." -- buried in some application note somewhere for the ADF436x PLLs there's a note about (paraphrasing), "You really ought to have both outputs, V+/V-, terminated in the same load since, umm, if you just ground V- and use V+ in certain corner cases the PLL will fail to work..." -- Rather than try to figure out whether or not I had one of those corner cases, I added the second resistor. :-) ) ---Joel | |||||||||||||
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>current-mode output. On the Analog Devices evaluation board schematics, they
>usually take the outputs through a fully differential LC filter for
>anti-aliasing and then through a balun to end up with a single-ended output.
>This of course takes a few more parts than immediately taking the output
>through a balun and then using a traditional single-ended filter. I've been
>debating the pros and cons of the two approaches and so far the reasons I can
>think of for going differential filter and then the balun are...
>
>1) Somewhat more "ideal" filter in that a ground plane (or worse, ground
>trace) isn't being used for return currents, it's just current flowing from
>one side to the other of the differential output. (Seems minor, although from
>past filter designs I've done at UHF this might be a much larger improvement
>than I'm guesstimating here.)
>2) Filtering out the high-frequency scunge before it hits the balun allows the
>balun to not handle quite as much energy and hence perform a little bit better
>(more linearly). (Also seems minor.)
>
>Am I missing anything else here?
>
>Thanks,
>---Joel
>