LVDS or RS 485 for lowest EMI

Typical LVDS drivers have much faster edges, which means that you will see EMI going up to hundreds of MHz - I speak from experience. In principle use of a twisted pair should mean cancellation of most of the EMI, but in practise driver skew and cable asymmetry mean that the cancellation is not perfect. If EMI is an issue, I'd go for the slowest edges I could get away with ...

Dave

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As serial connection on desktop scale (few meter), what would give the lowest electromagnetic interference, LVDS or RS485 ? The speed is few 10kHhz to few 1 MHz. There are no GND issues.

LVDS is 3mA into 100 Ohms while RS485 is 5V into 100 Ohms. Since LVDS is much faster, I'd assume it to have less but shorter transients.

Rene

I believe I've seen RS485 transceivers with controllable slew rates. Don't remember where though (Maxim? Linear Tech?).

Hi Rene, I just found this, while lookng for something completely different....

martin

After the first death, there is no other. (Dylan Thomas)

Thanks Martin, the article was quite an eye opener. How do I communicate the increased cost for the better cable...

Rene

I only read it quickly, but there was no mention about cable emissions with high data rates.

Time for a bit more digging around. But CAT5/6 etc. is installed everywhere, it cant be that much of a problem, can it?

martin

After the first death, there is no other. (Dylan Thomas)

More papers found :

Extending SPI and McBSP With Differential Interface Products

There is tons of documents after you have an idea.

Rene

It was less about the emissions of bad cable but losses. The common perception of RS485/422 is a mile at high baudrate, so one is fine in all cases. Not so. For longer cables and higher datarates, better cable are required. Yes, there are CAT5 and CAT6 cable. Installed everywhere ? No. I have them to become installed. Your paper also compared different drivers, suitable for different applications.

The paper

Rene

After reading through the other posts, it occurs to me that you could limit the slew rate of fast LVDS edges using series resistors and/or shunt capacitors at the driver end.

Also, LVDS usually has very low skew and is often more differential (as opposed to complimentary), so it might get better cancellation in a twisted pair.

Given all that, I think LVDS would have lower emissions. But this is just a gut feeling. I don't have tons of EMI experience or anything like that.

Whichever standard you use, you may want to use a common-mode choke to put a damper on any high-frequency common-mode signal that may be trying to get on the wire.

One of the reasons fast Ethernet and Gigabit Ethernet work with unshielded twisted pair is that they employ multiple common-mode cancelling schemes, including common-mode chokes. Since you need to be DC-coupled, you can't do all that, but you could at least use the common mode choke.

--Mac

Larry Brasfield wrote: [...snip non-informational gibberish...]

Slew-rate limited RS485 does not come close to achieving a "few MHz" bit rate, so once again you are a clueless p.o.s. shooting your mouth off with a lot of terminology you don't know a thing about. You are a worthless sack o'shyte as usual. Go away and stay away.

The OP stated "The speed is few 10kHhz to few 1 MHz."

Maxim has more than a few RS-485/422 transmitters that do 2.5 MB/S. Some folks, not so intent on spew as you, would call that close.

No, for the Nth time.

Close is not good enough- it either makes or it doesn't- period. You are a half-assed p.o.s. And do they have transceivers that do 2.5Mbps- you don't say?- what a fountain of the mundane you are, idiot.

All you are doing is publicizing your ignorance and mediocrity- that much should be clear even to you by now. You have nothing to offer usenet in the way of substantial posts- you are too damned dumb to understand the OP's question 99% of the time and then your answers are worthless garbage- just like you.