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0 pointsjbay8084y ago0 comments

I think you "well actually"'d a bunch of things I specifically didn't say.

I use "Johnson limit" synonymously with "thermal noise limit" because they are the same, and it's the limit of how low noise can be after removing all other sources of noise.

Most people, if they even learn about resistor noise, will only learn about thermal noise. If they're lucky enough to identify a resistor as the noise troublemaker in a circuit, they might not have any idea that they can potentially cut the noise 1000x by changing the resistor from thick film to thin film, with no other design changes, at the cost of one cent. It's not common knowledge, as illustrated by the fact that an article like this, dedicated to the topic of resistor noise, doesn't even mention it. And instead laughs at someone even considering to look for a "good" resistor as though it were superstition.

In fact, thick film resistors are far more common, so if you're in the situation where you're reading this article because you have a noisy resistor in your circuit and don't already know about Johnson noise, you almost definitely don't know about current/flicker noise, and since you got here because of a noisy resistor in the first place, a "good" thin film resistor is overwhelmingly likely to be the cure.

I'm not advocating ad hoc selection of components to reduce noise any more than selecting ad hoc components to reduce cost. A noise analysis can help you find the problem but won't help you solve it if you think your only option is to change the resistor's value, rather than its type.

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ldarby4y ago

Elsewhere in this thread there's a link to https://dcc.ligo.org/LIGO-T0900200/public, which backs up what you're saying:

"Metal film or thin film resistors have little excess noise in these tests and thick film resistors show large excess noise"

exmadscientist4y ago

Yep. Any time I actually care about the value of a resistor (which is to say, for precision analog work, not power supplies or logic pull-ups or LEDs or ...), I'll pick a cheap thin-film part, even without doing analysis. This does several things for me:

1. I get a "pretty good" (i.e., >80% of best achievable performance for <<20% of the effort) resistor in my important location.

2. I get a separate BOM line item for that resistor, so it stands alone (or with similarly important parts) and is easy to single out to, say, ensure correct ordering.

3. The part will be physically distinctive, so I can also distinguish it on the circuit board. If I have space, I'll even go up one size, to make it impossible to swap out with the less critical parts. (It would work fine, if expensive, to put thin-films everywhere. The reverse is not true.) It also sends the "pay attention to this" signal to anyone down the line who only has the board to work from.

I favor Susumu thin-film resistors, as they're generally cost-competitive, very high quality, available in very tight tolerances if needed, and visually easily identifiable as Susumu parts (though the series is not identifiable). This means they have to be thin-film, because Susumu doesn't make anything else. RR and RG series are the go-to parts here. If cost is a concern, Yageo RT is also excellent, but not as noticeable on the board. For analog work, I'll go no smaller than 0603 (1608M) to ensure values are printed on the parts. This pays for itself after only one gain resistor mixup on the bench....

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