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Sharing decoupling capacitors
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ogden:

--- Quote from: Zero999 on May 17, 2019, 08:02:09 pm ---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.

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Nice example. Slew rate of OP07 is 0.3 V/us. Decoupling of such a slow opamp is useless waste of capacitors :)
iMo:

--- Quote from: schratterulrich on May 17, 2019, 05:19:55 am ---You can find it at https://leiterplatte.jimdo.com/pdn-sim/

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I've been searching since ever after a tool which imports the Eagle PCB file and calculates impedancies/capacitancies/inductancies/resistancies of the top and bottom tracks.
OM222O:

--- Quote from: ddavidebor on May 17, 2019, 08:12:02 pm ---
--- 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.

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Is this a commercial product that needs to pass emi testing?
How many layer is your board and what stackup do you have?

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It doesn't require EMI testing and it's a simple 2 layer board. I can see people are at quite a large disagreement. Some say decoupling isn't even required and some say it's not done enough / properly. I guess I have to test it by removing the caps and checking with the scope to see the differences?
thm_w:

--- Quote from: OM222O on May 17, 2019, 09:46:35 pm ---It doesn't require EMI testing and it's a simple 2 layer board. I can see people are at quite a large disagreement. Some say decoupling isn't even required and some say it's not done enough / properly. I guess I have to test it by removing the caps and checking with the scope to see the differences?

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It depends on the design and the requirements. A schematic or board photo might help a bit (it seems hard to see a case where you can't fit in one 0603 per IC).
But your plan is a good one, measure, remove the capacitor, measure again.

https://www.electronicdesign.com/boards/whats-all-muntzing-stuff-anyhow
T3sl4co1l:

--- Quote from: Zero999 on May 17, 2019, 08:02:09 pm ---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.

https://www.eevblog.com/forum/projects/sharing-decoupling-capacitors/?action=dlattach;attach=739044;image

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There is AC in the circuit whether you intend there to be or not!

Run that from some longer cable for example, and the opamp is likely to oscillate.  Dynamics exist whether there is an apparent (large scale) signal present or not!  Thermal noise is sufficient to push a system over the edge from an initial resting position (which may accidentally be an unstable point).

This, incidentally, is why it's often hard to get SPICE oscillators to start.  There's no noise unless you've specifically placed noise sources.  And even so, if the amplitude is below the tolerance settings, it may simply be ignored and smoothed over.

And so, this is the other intent behind good decoupling -- to isolate the on-board network from the outside world.  And to do this, resistance is necessary.  That is why we dampen or "swamp" resonances with ESR, and often it's as simple as a 100uF electrolytic somewhere. :)

Tim
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