[sdiy] Nice info on decoupling caps

[ASSI]

On Thursday 19 January 2012, 11:54:04, David G Dixon wrote:
> I found this article to be frustratingly terse.  "Many ICs have internal
> circuitry which generates HF noise on the power rails" doesn't really tell
> me anything!  Am I to believe that an opamp generates HF, just because?

Surprisingly many of them do, especially if they are set to provide unity 
gain, but are not compensated for it.  You might not even notice...  Also, 
the PSRR of an opamp at RF frequencies is essentially zero, any RF present 
at the supply will propagate to the output.  Conversely, any RF present on 
the output (if you plug in an "antenna") will propagate to the supply.

The bypass does two things: short RF coming from the IC to the supply and RF 
coming from the supply to the IC (try putting a cellphone next to the 
circuit).


Regards,
Achim.
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