#### David_

##### Advanced Member level 2

This question relates to a(or all) hardware design of mine but it is partly simply a "I want to know why or what" situation in that all datasheets for ICs tell you where and what value and what kind of decoupling capacitor/s to use or in which range, according to the guidelines found on the web I should be fine as it is(power supply decoupled/bypassed with both high(10µF) and low(100nF) caps and then a 100nF cap at every IC.

I am thinking about a situation in which there are a high frequency SMPS followed by some analog and digital circuits on the same PCB separated as good as possible but the circuit does after all consists of both analog(linear regulator, op amps, voltage references, data converters) and digital(a µC & LCD stuff like that). The microcontroller(µC) communicate with the data converters with high-speed SPI ≈ slave1 = 5MHz, slave2 50MHz.

The SMPS is actually a low-frequency design(20kHz) but the µC does run with an internal oscillator of 32MHz, so the SPI 50MHz is below that(I am prototyping with a 86MHz controller(arduino) while are going to use a XMEGA in the end).

I'm sure one could say that to be sure I could but on every analog IC 2 or 3 decoupling caps of different values which is not specific but I want to know specifically.

If I have (and by the way I have so far meant ceramic capacitors, X7R and alike) a 10µF & a 100nF in parallel to decouple a IC, how can I estimate/work out/simulate what the frequency specifications that combo(or any other combo) could be expected to have and how do I take anti-resonance into consideration and is it relevant to do so?

How does I know what range of frequency's a capacitor value attenuate given that it has "appropriate standard" values of ESR and ESL?

I guess I want to look at how capacitors of different values/package sizes combine to decouple AC currents and get a understanding about this subject in general. I have acquired a Matlab student license and I am learning to use LTspice but I am unsure about what mathematics I need to know about or what simulation tools could be used to simulate this stuff.

I have a Analog Discovery or a set of scope/function generator to use to look at these capacitor combinations but they does not enable me to view small enough voltages but foremost they fall short in bandwidth so much its no use to even think about doing any practical evaluation other than low frequency.

I think that in practice this is not necessary to go into detail about but I really want to know about this stuff.

And about anti-resonance, there ESR and ESL is more important to be specific about but how do you even measure such values at say >10MHz. Is it even possible to prove any specific value at that high a frequency or is that just mathematical models that I one need to use and trust? In component datasheets one can see those diagrams depicting impedance and reactance changes over frequency and they range very high indeed.

Regards