Paralleling small value caps with large value PS caps

[jacco vermeulen]

Originally posted by Eva
in a different spatial plane from the main PCB

Eva,
can you explain this a bit more, please.

[jackinnj]

Originally posted by Eva
The resulting system from paralelling very dissimilar capacitors is going to resonate like crazy, usually at several hundred Khz, except when a resistor of high enough value is inserted to isolate one capacitor from another.

here's my experience from ham radio-dom -- all amplifier wiring (well, all wiring) can be a "radiator" or an "antenna" -- the decoupling capacitors on the power supply and at the point of application terminate the antenna into a low impedance load --

if you fail to properly terminate the RFI can cause bizarre behavior in opamps and discrete devices -- the folks at Analog Devices describe an amusing story of how a hand-held 2 meter (144 MHz) transceiver caused havoc with some instrumentation amplifiers which had not been properly bypassed.

Eva's example of a pulsed V or I application is certainly correct in a high efficiency SMPS -- I think that it breaks down for a Class A or AB1 amplifier driven from the mains --

we are now at that point in the life-cycle of SMPS designers (roughly coinciding with the advent of the IBM AT in 1980) to see whether there is an outbreak of mass insanity resulting from the failed efforts to stanch all instances of EMI, RFI etc.

[Eva]

Well, if the antenna is terminated into a low impedance in the capacitor bank side, won't we still have picked up RF at the other side in the amplifier module?

Note that the major source of high frequencies inside an audio amplifier is the output stage when it oscillates or a considerable amount of trebble is being amplified (requiring sharp base or gate drive pulses at crossover, that should be confined in a tight loop).


jacco vermeulen:

The principle is as simple and stupid as it looks: placing small signal circuits in a different plane, perpendicular to the power circuit plane, reduces induction effects.

[jackinnj]

Originally posted by Eva
Well, if the antenna is terminated into a low impedance in the capacitor bank side, won't we still have picked up RF at the other side in the amplifier module?

That's why we use 100nF on both ends -- at the capacitor bank and at the opamp or DAC or ADC.

[jacco vermeulen]

Originally posted by Eva
The principle is as simple and stupid as it looks

Eva,thank you.

That you can not have parallels in different planes i already guessed.
There is still a need if the boards are already separate and at distance from eachother ?

[Eva]

As the distance is increased, the various high current loops tend to turn themselves into a magnetic multipole mess that nearly cancels itself. In the very near field there is not such an effect.

[maxlorenz]

here's my experience from ham radio-dom -- all amplifier wiring (well, all wiring) can be a "radiator" or an "antenna" -- the decoupling capacitors on the power supply and at the point of application terminate the antenna into a low impedance load --

Thanks for that one, jackinnj.
It worked for me


Mauricio