Typical Parasitic Inductance Introduced by 0402, 0603, 0805, 1206 Resistors - Page 1
Forum section: electronics / projects

author: niconiconi
author_rank: Frequent Contributor
timestamp: March 21, 2020, 07:55:15 pm
content: Is there a reference guide for parasitic inductance introduced by common SMD thick film resistors like 0402, 0603, 0805, and 1206? Specifically, I'm using resistors as series terminators or jumpers on >100 MHz digital signals, and I want to quantitatively compare how different resistor packages affect the signal integrity. The motivation is to answer this question: which has more inductance? Two standard 0.3mm vias on a four-layer PCB, or a 0402 resistor in the middle?

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2020, 09:05:59 pm
content: They don't just have inductance, but also capacitance. Which dominates, depends on the resistor value.It really easier to think about the wavelength of the signal vs the physical size of the components. 100MHz is a wavelength of about 3m, which is massive, compared to those components, so it isn't an issue.

author: Someone
author_rank: Super Contributor
timestamp: March 21, 2020, 09:46:01 pm
content: There is substantially more to it than just case size:https://www.vishay.com/docs/60107/freqresp.pdf

author: niconiconi
author_rank: Frequent Contributor
timestamp: March 21, 2020, 10:52:13 pm
content: Quote from: Zero999 on March 21, 2020, 09:05:59 pmIt's more of a inquiry out of curiosity.Also, 100 MHz digital signals are the signals with subnanosecond rise and fall time, so any parasitic must have an effect. Although in practice they are minimized by continuous microstrips and all reflections are mostly absorbed by resistive termination, but anyway, I guess some knowledge about their properties can help in practical applications.Quote from: Someone on March 21, 2020, 09:46:01 pmGood information. But I'm more interested in thick film resistors and their performance limitation, i.e. insignificant general-purpose SMD resistors with limited performance, not thin film precision RF resistors that worked up to 10 GHz+ as shown in this appnote. There must be some experimental data somewhere. Perhaps I have to dig harder at Google Scholar.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2020, 11:06:18 pm
content: The general underlying fact is the component is a conductive element continuing the transmission lines of the connecting traces.So, whatever the width and height of the conductor over the substrate is, that's your transmission line impedance, and component length is line length.So, bigger components have a proportionally lower cutoff frequency, and wider components have a lower Zo.What does Zo do? The ratio between rated value and Zo is relevant. The closer they match, the nearer to 1 the ratio is -- the higher the cutoff frequency will be. The more mismatched they are, the lower the cutoff will be -- and approximately proportionally so.So a 100 ohm resistor will be a good match to most traces, and itself over the substrate, while a 10 or 1k resistor will have about a 10x lower cutoff.Which kind of cutoff will it have? R < Zo, inductive; R > Zo, capacitive. That is, for lower impedances, the equivalent inductance dominates, while for higher impedances, the equivalent capacitance dominates.We don't need to do any messy transmission line calculations if we're only after the low-frequency equivalent:ESL = mu_0 * Z/Zo * lengthCp = e_0 * k * Zo/Z * length(assuming substrates with mu_r = 1, and some dielectric constant k, the usual case)Where mu_0 ~= 1.257 uH/m, e_0 ~= 8.84 pF/m, Z is the transmission line impedance and Zo is the impedance of free space, ~377 ohms (also Zo = sqrt(mu_0 / e_0) if you prefer reducing it in terms of just the two constants, but this is easier to remember I'd say).Including the crazy high frequency range documented in the Vishay document, you'll have to consider full transmission line behavior, as well as width, height, material (the resistor body is usually Al2O3, while the substrate is whatever it is), higher order modes... radiating modes probably?, whatever. That's unsurprising, there's just so many more degrees of freedom for high frequency waves so we aren't going to get away so easily with a hand-wave up there.Tim