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Old 17th April 2012, 11:46 PM   #11
3n2323 is offline 3n2323  United States
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Quote:
Originally Posted by Alvis View Post
Small signal ground fluctuations on high power stage are better to have then high power ground fluctuations on a small signal ground.
thank you Alvis, sorry i don't understand.

if noise -> small singal -> low power ground fluctuations
output stage -> high power stage
then it's better to connect shielding to the ouput ground, instead of the input ground?

if noise -> large signal -> high power ground fluctuations
amp input -> low power stage
then it's bad to connect shielding to the input ground?

Quote:
The idea is to catch the radio waves and short circuit them to ground, before they have a chance to enter the system. (They turn into miniscule amounts of heat.
So the idea is to "shield" or put a "cage" around all sensitive amplifying components, and this cage needs to go directly to ground. Short to ground, the earlier the better.
understand that noise needs to be grounded, just don't understand why "the earlier the better". if we are going by power, then power matters, earlier or later doesn't, right?

i tend to think as long as the noise is grounded, the input net won't even see the noise, so why risk input ground disturbance for it? you guys have suggested where noise is grounded does make a difference, that is to say, if it is grounded to the output ground for example, the input net will still see it. so i'm now after the why part of this reasoning, after the actual mechanism that makes this difference, then i would understand why "the earlier the better", ground at the input that is.
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Old 18th April 2012, 01:19 AM   #12
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I think you're on the right track stating it as "ground/reference." A voltage has to have a reference. That's why you'll see many forum posts that discuss "star grounding". A schematic may show several ground points, and depending on board layout they may or may not be at the same potential, i.e. there may be a voltage potential between two points, although each are "ground." That sort of potential is what is avoided between the input and "noisy shield" by connecting it as has been discussed here.
Hopefully someone will confirm that question of "Is it common-mode at the input?"
Common-mode situation refers to a signal (or noise) that is present on both amplifier inputs simultaneously. If an amp only applies gain to a difference between the inputs, a common-mode signal is rejected, and is spec'd as Common Mode Rejection Ratio, CMRR. The actual ratio is the open-loop gain divided by the common-mode gain, since nothing is perfect.
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Old 19th April 2012, 11:33 PM   #13
3n2323 is offline 3n2323  United States
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Quote:
Originally Posted by sofaspud View Post
Common-mode situation refers to a signal (or noise) that is present on both amplifier inputs simultaneously. If an amp only applies gain to a difference between the inputs, a common-mode signal is rejected, and is spec'd as Common Mode Rejection Ratio, CMRR. The actual ratio is the open-loop gain divided by the common-mode gain, since nothing is perfect.
most interesting, thank you sofaspud!
i wonder why noise won't have the same influence on both termianls of the input, anyways, one more topic on my reading list
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