Electronics > Projects, Designs, and Technical Stuff
Sharing decoupling capacitors
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schratterulrich:
In the past I was often unsure about the right decoupling capacitors.

We then introduced a software in the company that simulates the impedance of the power delivery network. That made things easier.
Depending on your transient current consumption and your voltage ripple allowed you can say that e.g. 100 mOhm up to 100 MHz would be sufficient.

As mentioned by Tim you can analyze the decoupling caps and planes, so I have written my own software.
It's a big help for supply systems with power planes but maybe it can help in your case too
You can find it at https://leiterplatte.jimdo.com/pdn-sim/

The result can look like this


Best Regards
Ulrich


ogden:

--- Quote from: T3sl4co1l on May 17, 2019, 02:03:23 am ---Decoupling is overblown.

--- End quote ---

If you consider signal integrity, then indeed. Most "2-layer" circuits will continue operation without single decoupling capacitor. On the other hand small & seemingly innocent uC without proper decoupling can trash radiated EMC tests easily.
Zero999:
You quite likely don't need a decoupling capacitor for every IC, especially those which sit in a steady state.

I've just built a circuit with no decoupling and it works perfectly. It's a voltage reference to give 5V and 10mV out for adjusting a DC amplifier. It's very simple, just LM4040-N-2.5 and current limiting resistor, connected to an OP07 op-amp with a gain of two and a potential divider. The output voltages are trimmed by adjusting a couple of potentiometers. There is no other high speed circuitry on the board or nearby and is powered by a 15V linear power supply module, so no decoupling capacitors are required, because there is no AC in the circuit.
ddavidebor:

--- Quote from: OM222O on May 16, 2019, 11:01:09 pm ---I think I've been using too many decoupling capacitors in my designs, with each chip getting a 100nF decoupling capacitor of its own.
sometimes two chips are as close to each other as physically possible, for example: an op amp and a voltage reference both powered from a 5v supply. A lot of times the chips work well even without any decoupling like Dave showed in his megatron computer video. I was wondering if two chips that are close to each other (usually SOIC-8, so very very close) can share the same 100nF decoupling cap or if I should bump it to let's say 1uF for good measure if I decide to make it shared.

--- End quote ---

Is this a commercial product that needs to pass emi testing?
How many layer is your board and what stackup do you have?
iMo:
Decoupling is important and usually ignored. Any today's mcu has got 5ns edges. Even a blinking with an led produces transients.

And even a fully quiet DC circuit can go easily weird when irradiated by an EMI.

A friend of mine is running an EMC lab and even BIG players sometimes come with something which does not pass the basic test because of a missing 1cent capacitor.

That would apply to the above 5V reference source too :)
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