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Old 21st July 2005, 06:58 PM   #1
Reina is offline Reina  Germany
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Default 74hc or 74ac logic chips for digital audio ?

Hi,

the older high-end equipment uses 74hc.. logic chips for digital audio signal processing. it is a good idea to use 74ac.. today? wich is better in noise performance ?

greetings.
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Old 21st July 2005, 07:01 PM   #2
tubenut is offline tubenut  South Africa
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Dr H recently got rid of the AC chips in his dac for something else and reports + results. Maybe he will fill us in.
You could search....
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Old 21st July 2005, 07:32 PM   #3
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Default How many times do we have to say this..........

Do not use AC logic. Some will say you can..........do a search...about 2-3 months ago (I think)...........an expert was in the process of designing a radical new clock, and the usual entertainment followed.

Jocko
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Old 21st July 2005, 08:12 PM   #4
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Good answer Jocko

I tend to agree with you. But ............
Do you have any explanation why AC's are the slime of the earth?

All the best from

Lars
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Old 21st July 2005, 09:37 PM   #5
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Dig out yur old copies of EDN......early 90s or so......

They did a series of articles on ground bounce and "slime of the earth" logic.

One of my less-than-famous audio designer buddies talked me into using it once. He was infatuated with the symetrical rise and fall times.

Took me a while to decide if Phred hated what I put it into because he was just being Phred, or it really did suck. I eventually figured it out............

Jocko
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Old 21st July 2005, 09:48 PM   #6
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Jocko: Is it that problem with high switching speeds, combined with the VCC and GND leads inductance causing malicious switching of the output?

http://www.fairchildsemi.com/an/AN/AN-680.pdf
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Old 21st July 2005, 10:40 PM   #7
gmarsh is offline gmarsh  Canada
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Noise performance? HC, HC, HC. And if you need faster than HC, use VHC.

AC logic isn't as bad as Jocko claims... most semiconductor companies still make the family, which indicates that there is some use for it in the world. I use it for a couple of things, though not in audio stuff, or any type of analog application for that matter.

AC logic can beat the hell out of your board's power supply rails and can create an ungodly amount of EMI if you're not careful with it - and by careful I mean paranoid - use a TSSOP package, two decoupling caps, a ferrite, and series R's on all outputs.
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Old 22nd July 2005, 01:22 AM   #8
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I'm not the only one who thinks it is terrible. The chip companies came out with several new families in response to the ground bounce problem.

But, as you point out, series R can help. It is also useful on slower stuff like HC, as it keeps the peak currents down. That is key to reducing EMI. The trick is to make the R large enough to limit the peak currents, but not so high as to slow down the waveform. Slower waveforms spend more time in that no-man's land when the output transistors go through the transistion stage. The PSRR at that point is only 6 dB. (Someone asked about that earlier.)

Jocko
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Old 22nd July 2005, 07:10 AM   #9
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A very good, comprehensive paper on the topic:

http://www.fairchildsemi.com/an/AN/AN-640.pdf

An excerpt:
Ground bounce is an unwanted noise source that is found
in most logic families available today. Due to increased
edge rates and voltage swings, ground bounce can be
more of a problem with new Advanced CMOS logic families.


As tubenut says, I replaced AC logic with HC logic in a Theta DAC and found the overall presentation to be more relaxed, less edgy, quieter, easier to listen to.

Ryan
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