66 lines
3.9 KiB
Markdown
66 lines
3.9 KiB
Markdown
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# 为什么 RS485 差分总线电路中需要使用共模电感
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这个问题来源于差分通信本身就可以抑制共模干扰，为什么还要“画蛇添足”的增加同样是抑制共模干扰的共模电感。
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一个论坛上的回答比较中肯，原文如下：
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[Why common-mode choke on differential-signal serial lines?](https://electronics.stackexchange.com/questions/587771/why-common-mode-choke-on-differential-signal-serial-lines)
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I have 2 questions:
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1. Why using a common-mode choke on serial communication lines (like
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CAN, RS485) if the signal is differential? Wouldn't the common-mode
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noise simply be cancelled in the receiver?
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2. I reviewed a project where the CAN had a common-mode choke, but the RS485 did not. Is there a technical reason for that?
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Thank you
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---
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```
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Wouldn't the common-mode noise simply be cancelled in the receiver?
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```
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A lot of noise would be heavily cancelled in a differential receiver but, high frequency stuff can cause problems and, of course a CM choke acts like an inductor and hence increases its series impedance with frequency. High frequency noise beyond the bandwidth limitations of the receiver can still wreak havoc.
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```
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I reviewed a project where the CAN had a common-mode choke, but the RS485 did not. Is there a technical reason for that?
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```
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There may be but, the devil is in the detail (and the performance testing and the grounding of this and that and what spec it's being tested to and the data bandwidth and the chips in question.......)
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```
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What's your take on modern transceivers like MCP2561FD using a "SPLIT" output pin for the termination (see page 6), supposedly for common mode stabilization? Reading the datasheet they seem mostly concerned about radiated emissions at higher baudrates (CAN FD) and not so much about radiated/conducted susceptibility. –
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Lundin
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Sep 22, 2021 at 14:07
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The link and the data sheet don't really say much about it. The data sheet covers two versions; one with and one without so there's not much to go on really @Lundin - maybe raise it as a new question? –
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Andy aka
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Sep 22, 2021 at 15:44
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I posted a question about this pin over at Codidact here: CAN "split" pin, bus termination and common mode stabilization. –
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Lundin
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Sep 24, 2021 at 9:22
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```
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---
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The differential receiver has a limited range of common mode rejection, on the order of volts. It cannot remove common-mode noise peaks whose voltage exceeds that level. The common mode choke transformer increases that range for high frequency pulses, using two techniques:
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1. It converts some of the high frequency noise (whether common mode or differential) to heat (unlike a transformer, the ferrite in a common-mode choke is purposely lossy
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2. It subtracts the common mode signal (as a coupled inductor) allowing 10's of volts of high frequency noise difference between the two grounds at the transmitter and the receiver
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In answer to your question on why you saw a common mode choke in a CAN bus port but not in a RS-485 port: that's just a decision that those particular engineers made. It has more to do with economics and little to do with differences between CAN bus and RS-485.
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---
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A common-mode filter will block noise induced on the pair (and its reference ground) from making its way into your system, even if the differential receiver rejects it. In other words, the CM filter prevents EMI/ESD related system upsets in the presence of strong noise.
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Even Ethernet, which not only uses differential signaling but also transformer isolation, can benefit from suppressing common-mode noise.
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As to why it was applied to CAN bus and not RS-485 in that particular system, I suspect that the designer felt that the places the CAN bus connected to were more likely to have strong electromagnetic noise (like from ignition, fuel injection solenoids, and other power train sources) than the places RS-485 went (in-cabin only.)
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---
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总结来说就是增加共模电感可以更好的抑制干扰，比如说高频的干扰。
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