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Re: [avr-gcc-list] AVR ADC (waaaaaaaaaay off topic)

From: Graham Davies
Subject: Re: [avr-gcc-list] AVR ADC (waaaaaaaaaay off topic)
Date: Sat, 14 Apr 2007 09:30:38 -0400

Clemens Koller wrote:
For a propably [sic] "better than okay" solution I would recommend a
100pF C0G or even a more priced 1nF C0G capacitor in parallel to
[a 100nF X7R ceramic], close to the pins, to be very sure to get rid
of any high frequency components.

Personally, I think Benjamin is right to be cautious about "common sense" and to know his limits.

What you're recommending used to be "common sense", but doesn't actually work. If you draw out the equivalent circuit of two different valued ceramic capacitors connected in parallel you will discover you have a parallel reasonant circuit. For simplicity, draw the series parasitic inductance at the top, away from ground, on both capacitors. Now consider frequencies at which the larger capacitor is effectively a short circuit. Look at what's left - a parallel (anti-) reasonant circuit with the power supply tapped into the middle of the inductor. Near the reasonant frequency, the impedance flies up, which is the opposite of what you want.

These days, "common sense" for high frequency decoupling is to use capacitors of the same value everywhere and concentrate on getting the lowest possible parasitic inductance between power and ground. The parasitic inductance of a ceramic chip capacitor depends largely on the physical dimensions of the chip and isn't much affected by the capacitance value.

None of this really has much bearing on decoupling the voltage reference of a microcontroller. The original 100nF ceramic chip plus good circuit layout will be fine.


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