[sdiy] Nice info on decoupling caps

[Oren Leavitt]

On 1/19/2012 3:01 PM, ASSI wrote:
> 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.

 > (try putting a cellphone next to the
 > circuit)

Your circuit will become a telephone ringer when a call comes in.
zit-zit-zit...zit-zit-zit...zit-zit-zit...

A grid dip meter can sniff out some interesting noise that might 
otherwise go undetected.

- Oren