When adding by-pass capacitors to a design, I have read that it is not enough to just place the typical 0.1 μF value and the 10 μF one; you have to check why you need those values. I mean, which are the frequencies in your system, in your PCB, that can disturb the design.

My design would be something like:

Signal 🡆 Voltage follower (op-amp) 🡆 DAQ

In the power supply lines of my op-amp I have to add by-pass capacitors to filter two noises: the one that comes from the switching frequency of the power line (I checked in the datasheet that it is 500 kHz) and the 'switching' frequency that comes from sampling time (DAQ).

In spite of this, I am at a bit of a loss in term of choosing good by-pass capacitors (capacitance, dielectric and package) and specially in how to check switching frequencies or frequencies to filter in the datasheets of the components of my circuit (which term do I have to search in the op-amps', DAQ and other components). Filtering for 500 KHz, for example, and checking real curves impedance-freq does not give a clue of which capacitor I have to use (they all filter for higher ones).

Apart from this, if I am filtering in order to achieve a stable behaviour of the buffer, would I need an RC circuit in the output before the DAQ? Which would be the difference?

I cannot find either a capacitor that could filter properly in my frequencies (500 kHz). Here it is a plot from KEMET capacitor simulator from one example.

enter image description here

  • \$\begingroup\$ You want specific filtering which differs from simply adding bypass caps. Determine how much noise your power supply generates and how much noise is acceptable to your analog circuits. The op-amp has a degree of tolerance specified as PSRR. Say your power supply has 200mV ripple at 500kHz and you want 1mV max for your analog circuit. That would suggest you need a low pass filter with 46dB attenuation at 500kHz. You can then investigate filtering options to achieve that and it won’t simply be a capacitor. \$\endgroup\$
    – Kartman
    Commented Apr 27, 2023 at 13:33
  • \$\begingroup\$ @Kartman I have some doubts. How did you calculate the 46dB attenuation? How do I calculate noise? So, for example. I can design an RC circuit, that would give t as time constant value or frequency, but, how do I calculate an RC for attenuation and how I get the attenuation value I need for that calculation? \$\endgroup\$ Commented Apr 28, 2023 at 5:42
  • \$\begingroup\$ In my example the attenuation was 1/200 or -46dB. dB volts = 20 X log10(1/200). As for noise, you need to qualify and quantify - what noise? And how much? Measure it. Then what do you want to decrease that noise by? Then consult online filter calculators. Then simulate. \$\endgroup\$
    – Kartman
    Commented Apr 28, 2023 at 12:50
  • \$\begingroup\$ @Kartman I mean, if the IC that gives me the signal to read, gives it to me with 3mV of ripple before the buffer i should have to design an RC with that attenuation in the output freq of the IC, right? Ex. IC output freq of 20kHz, so find an RC circuit that attenuates -46dB my signal at 20KHz (so the Fcutoff would be lower than 20KHz for that RC). \$\endgroup\$ Commented Apr 28, 2023 at 12:51
  • \$\begingroup\$ Que? What IC, what signal? We were talking about noise and ripple on the power supply. \$\endgroup\$
    – Kartman
    Commented Apr 28, 2023 at 12:53

1 Answer 1


Actually, that's a bit misnormer. Sure the frequencies of the system is important if you want to bypass at that frequency. But that's not all.

For example, if you take a simple square wave generator circuit. Even if you have it running at 0.1 Hz, that's really not the frequency you have to worry about as it's almost DC.

However even if the edge of the square wave happens very rarely, the edge of the square wave can be very fast. In theory the mathematical edge is infinitely fast so it requires infinite amount of energy in no time at all to switch.

In real life let's assume it can happen in 1 nanosecond. If you have a modest amount of wire and it drives some inputs, the chip would have to drive say 50pF of capacitance from 0V to 5V in 1 nanosecond. It needs a current pulse of 250mA for 1ns to achieve that. So that's a lot of current needed and it is needed fast, meaning there's high frequencies.

The point of getting that large current means that the wiring and capacitor must have low resistance, as resistance limits how much current is available.

The fact that the current must rise to 250mA in much less than 1ns means the wiring and capacitor must have low inductance, as inductance limits how quickly the current is available.

This gives a rough understanding why a 100nF plus 10uF capacitor may not have any effect at high frequencies, because you can't fit large capacitors close to chip, and you need capacitors with small physical size to reduce inductance, and small physical size caps also come with smaller capacitances, which means smaller capacitors (both in size and capacitance) are better at higher frequencies.

So, cap types.

If you want very large capacitance for bulk storage, it might be electrolytic, but those are not good at high frequencies, or cold.

If you want a simple bypass cap, large and small capacitances are available in many Class 2 dielectrics, such as X7R. They just have large tolerances, large temperature coefficiet, and even large voltage coefficient. A 10 uF 50V rated capacitor may have 10 uF capacitance at 5V, but it may have only 5uF at 40V.

Class 1 dielectrics are most stable and have good tolerances, but are small, from few pF to few nF. But for making analog filters or crystal load caps, you definitely want NP0 type.

Sure, other types of caps exist, such as plastic film, low ESR electromytics, polymer electrolytics, tantalum, and they all have their applications, but also each cap type have their own problems or quirks.

  • \$\begingroup\$ Thanks for your answer. But it is not clear for me. First, I understand you are speaking about power availability (decoupling capacitors) and I am talking about by-pass capacitors. Yes, the same capacitor can handle both tasks but as far as I know, they are different things. And second, I cannot understand how to check the freq values I need to check or how to choose the specific values, dielectric and size for by-pass (without using 'standard-typical' 100nF) capacitors according to the real curve impedance-freq and its filtering implications. \$\endgroup\$ Commented Apr 27, 2023 at 10:53
  • \$\begingroup\$ @DevelopingElectronics what is the difference of bypass and decoupling caps to you then? And what are the frequencies of your example? The rest I can already fill in. \$\endgroup\$
    – Justme
    Commented Apr 27, 2023 at 11:06
  • \$\begingroup\$ I was told that decoupling was about isolate two parts of a circuit. For example, a decoupling capacitor would avoid voltage drop (being a battery). By-pass just filter noise, by-passing it to ground. The same capacitor acts in both terms, that is why they are usually named the same, but actually, they are different things. That was what I was told about. My freq are 200 kHz in the DAQ and 580kHz in the power supply. I can't find any model that his real imp-frq plot filter those values perfectly (minimum of the curve). Check ksim3.kemet.com/capacitor-simulation \$\endgroup\$ Commented Apr 27, 2023 at 11:39
  • \$\begingroup\$ You can also check this terminology thing in the "Solution" paragraph of this article: allaboutcircuits.com/technical-articles/… \$\endgroup\$ Commented Apr 27, 2023 at 11:58
  • 1
    \$\begingroup\$ @DevelopingElectronics Weird thinking no cap acts at 500 kHz. A generic X7R 10uF capacitor has impedance of 0.05 ohms at 500 kHz. A 47uF has 0.009 ohms. The more it has capacitance, the more effective it is at 500 kHz, but 100uF may be unuseful at 500 kHz if resonance peak is below 500 kHz. \$\endgroup\$
    – Justme
    Commented Apr 28, 2023 at 6:22

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