[SOLVED] Placement of decoupling capacitors on PCB trace

If one large capacitor says 100uF and a few small capacitors say 100nF and 100pF to be placed on a PCB trace connecting battery pad and a chip Vcc input pin, how would you deploy these capacitors?

Put them all close to chip Vcc input pin?
or put a large one near battery and small ones near the chip?

Why?
Any good document to explain this?

Thanks for your help.

Re: Decoupling capacitor

for a microwave chip, you want the 100 pf as close as possible, followed by the 100 nf right next to it. The placement of the largest cap is less critical, although if it is a high current device like a transmit amplifier or a low noise device like a VCO you would want the big one close by also.

Re: Decoupling capacitor

NE555 chips(timers) need a big C as close as poss, cause they cause a transient S/C across the Vcc line to earth.
Frank

Re: Decoupling capacitor

passerby said:

If one large capacitor says 100uF and a few small capacitors say 100nF and 100pF to be placed on a PCB trace connecting battery pad and a chip Vcc input pin, how would you deploy these capacitors?

Put them all close to chip Vcc input pin?
or put a large one near battery and small ones near the chip?

Why?
Any good document to explain this?

Thanks for your help.

Click to expand...

The "why" is because any trace/wire will create additional inductance (and resistance). When the device tries to pull a LOT of current very quickly, having the capacitors nearby means the stored charge int he caps is almost directly connected to the circuit desiring the surge of current. If you put some L (inductance) between the cap and device, then the change in current going through the inductance will be slowed... remember current through an inductor does not change instantaneously. Since the current through the paracitic inductance is slowed, the voltage at the device will being to drop as the available charge at that node is used up, and before new charge carriers can move from the capacitors, through the paracitic inductancor.

You want your smallest caps to shunt any higher-frequency signals to ground, that may be sneaking out toward your power supply. The larger caps are typically used for lower frequency instantaneous-current draw needs. Read the app notes of your device's spec sheet for the manufacturer's reccomended capacitor usage.