Electronics > Projects, Designs, and Technical Stuff
Isolated CAN ground connection
Mad ID:
Hi everyone.
In my current project I must use isolated CAN bus. Transceiver I'm using is from Analog Devices ADM3053 which includes an isolated DC/DC converter.
I'm confused by the datasheet (Figure 32) where a bus connector is depicted without a ground wire connection, i.e. only CAN+ and CAN- signals present.
I know CAN is differential but my opinion was that ground is needed especially with isolated CAN transceivers. (e.q. RS-485 standard clearly states that a ground wire should be present in the communication cable).
This is how I think it should be done: All CAN nodes have to be isolated except for one. This one provides the single-ground (earth) connection for the whole bus. Ground wire is carried along with the twisted pair to each node. No ground loop is created.
Please help me clarify this. Do I need a ground connection or not? Individual nodes are not! referenced to earth, if so isolation would be a waste of money.
Thank you!
Performa01:
Differential interfaces have a rather limited common mode voltage range. This is of course depending on the receiver, yet many CAN transceivers can handle just -2 to +7V.
On some sites, ground potential differences of several volts can easily build up over longer distances. Thus it certainly is a good idea to have all receiver grounds connected together by means of the ground wire of the communication cable. This does not create a ground loop since no signal is referenced to ground. That’s (one part of) the real beauty of differential pairs.
There might be some compensation current flowing through the ground wire, this should be low enough to not produce any significant voltage drop.
In noisy environments, shielded cabling is good practice anyway. This also helps to improve ESD immunity. The shield can only be effective when there is a low impedance path to the ground of the circuit that shall be protected. One single ground connection in an installation with several tenths of meters cable running is certainly not low impedance for higher frequencies. If galvanic isolation is really required for some reason, then each node should at least be connected to the shield by a HV capacitor (e.g. 10nF/2kV).
Well, that’s how I handle that very issue in my current project – it’s not proved in practice so far, as I’m still busy getting one single device working with all its features, so external connections over longer distances are designed in (CAN being one of them), yet of no relevance right now.
Hope this helps a bit to make up your mind! ;)
gxti:
That's a very neat part, It costs twice as much as a potted DC-DC converter in small quantities, but still damn cool. If you want to save a few bucks maybe look into using a seperate transceiver, isolator, and isolated power supply -- it seems to be a bit cheaper for small quantities. If you need a small footprint though, this chip can't be beat.
--- Quote from: Mad ID on March 18, 2012, 03:21:11 pm ---I'm confused by the datasheet (Figure 32) where a bus connector is depicted without a ground wire connection, i.e. only CAN+ and CAN- signals present.
I know CAN is differential but my opinion was that ground is needed especially with isolated CAN transceivers. (e.q. RS-485 standard clearly states that a ground wire should be present in the communication cable).
--- End quote ---
I'm not sure either. There's no mention either way in the datasheet other than that diagram, so I would go with your intuition and use a ground wire.
--- Quote ---This is how I think it should be done: All CAN nodes have to be isolated except for one. This one provides the single-ground (earth) connection for the whole bus. Ground wire is carried along with the twisted pair to each node. No ground loop is created.
--- End quote ---
I don't see much point in strongly tying the bus ground to anything. Use the isolated transceivers everywhere, and if you're feeling frisky, use a 1M resistor to reference bus ground to system ground at exactly one point, just to keep it from floating around.
Mad ID:
--- Quote from: gxti on March 18, 2012, 08:43:16 pm ---That's a very neat part, It costs twice as much as a potted DC-DC converter in small quantities, but still damn cool. If you want to save a few bucks maybe look into using a seperate transceiver, isolator, and isolated power supply -- it seems to be a bit cheaper for small quantities. If you need a small footprint though, this chip can't be beat.
--- End quote ---
Not sure if save is possible. The cheapest isolated DC-DC converted I've been able to find is 6$ (http://search.digikey.com/us/en/products/NTE0505MC/811-1679-5-ND/1927240) @ 500 pcs. On top of that I need a isolated transceiver which is at least 3$ (e.q. ISO3080 from T.Instruments).
With Analog Devices IC I have two completely isolated interfaces i.e. they have seperate power supplies and each can communicate with different remote equipment.
Please recommend a cheaper solution if there is one :) ?
--- Quote from: gxti on March 18, 2012, 08:43:16 pm ---I don't see much point in strongly tying the bus ground to anything. Use the isolated transceivers everywhere, and if you're feeling frisky, use a 1M resistor to reference bus ground to system ground at exactly one point, just to keep it from floating around.
--- End quote ---
Take a look at this app note from TI.
http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=slla268&fileType=pdf
Page 7, paragraph in the middle.
Thanks for your help.
Short Circuit:
The CAN transceiver has relative low imput impedance and a 2~3V recessive bus state, so it will always track the bus CM voltage.
If you read the datasheet, you'll notice there is no mention of common mode voltage range, which implies no ground connection.
Basicly the CM range is the 2.5kV isolation voltage, with a limit of 25kV/us voltage change rate.
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