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Design >> Analog Design >> onchip/offchip decoupling cap
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Message started by mixed_signal on Aug 8^th, 2012, 12:07am

Title: onchip/offchip decoupling cap
Post by mixed_signal on Aug 8^th, 2012, 12:07am

The frequency of my circuit is 30khz. Analog & digital has seperate Vdds but no seperate ground pins.Analog section has onchip decoupling cap of 15pf but digital has no decap.

Is offchip decoupling cap for analog and digital ok?

Title: Re: onchip/offchip decoupling cap
Post by raja.cedt on Aug 8^th, 2012, 1:14am

always on-chip decap is real decap, when you don't have on chip decap only off chip decap one and only the option. Mainly when you have decap off chip it is not cap, you have bond wire in series with cap which results in a series resonance ckt.

Imagine there is a frequency component at the resonance frequency, but inductor cancels the capacitance.

Thnak,s
Raj.

Title: Re: onchip/offchip decoupling cap
Post by BackerShu on Aug 28^th, 2012, 8:46pm

raja.cedt wrote on Aug 8^th, 2012, 1:14am:

always on-chip decap is real decap, when you don't have on chip decap only off chip decap one and only the option.

Hi Raja,
A little confusion about the last half of quoted part.
Do you mean only one, either analog or digital, should be chosen to be decoupled off chip? If so, what's the reason?

I thought both of them could be decoupled off chip, although it won't be as effective as on chip decoupling.

Title: Re: onchip/offchip decoupling cap
Post by aaron_do on Aug 29^th, 2012, 2:44am

Quote:

Hi Raja,
A little confusion about the last half of quoted part.

that's putting it lightly. Also, I don't think that's what he meant.

BTW, why not just assume some 1-2nH bonding wires, and simulate the circuit? See if there is any instability, performance degradation, or noise spikes. If your circuit has any possiblity of fast transients coulping to the power supply, then some on-chip decoupling would help.

regards,
Aaron