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I was wondering where I could find a list of interface which require differential pair or particular design rules ?

I know for example that USB require different pair routing. I identify them because their line are called D+ / D-.

However in another application I have a speaker and a mic which also have - and +, does this make them also a differential pair ?

chris
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  • depends on the speaker driver, if they are differential or single ended driving – KyranF Jun 08 '16 at 04:32
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    Unless you're dealing with signals in excess of several hundred MHz, I wouldn't worry too much about the intricacies of differential pairs. +/- just means plus and minus, differential signals will usually be part of some digital data line (but can be analog) and will often have a note somewhere saying "traces must have XX ohms differential impedance". Technically speaking any signal that's transmitted as both a positive and negative half is a differential signal, but at low speeds, there are rarely any special PCB trace routing caveats. (I'd keep the mic traces clear of noisy sources though) – Sam Jun 08 '16 at 06:18

2 Answers2

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Generally, differential pairs are used in high-frequency data lines, which are most prominent in devices used in sophisticated applications (memory buses, digital video, etc). And sophisticated devices tend to have associated literature to assist in the design where they describe which traces need to be routed as differential pairs.

What that means is you don't really need to memorize which kinds of devices do and don't require the special routing technique. You DO however need to always read the datasheets, design guides, and reference designs.

That's how the pros do it.

Dan Laks
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In addition to the comments and answers, there are a class of analogue applications where we use a differential interface.

This is usually where we have very low level signals, such as from a microphone, thermocouple or perhaps a strain gauge.

These interfaces are very susceptible to common mode noise and using a differential interface significantly reduces it (it does not completely eliminate it in practice).

These are not high frequency or high speed interfaces (thermocouple sampling rates are very slow as the device can only respond relatively slowly to changes in temperature anyway) but they are highly sensitive to noise.

Community
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Peter Smith
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