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CAN transceiver with high differential input impedance

Discussion in 'Electronic Design' started by Mochuelo, Sep 25, 2005.

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  1. Mochuelo

    Mochuelo Guest

    Hi,

    I posted a similar question some time ago, but that was about RS485.
    Today I discovered that CAN is much closer to what I want (since it
    includes CSMA/CD+AMP).

    Does anyone know of a CAN transceiver with megaohms of differential
    input impedance? I am ok with 1 kbps of speed, or even less, but I do
    need around 4000 devices connected to the bus. I haven't found any
    transceiver with more than 100 kohm of Rdiff, and I don't see any
    reason why they could not make them with tens of megaohms. I do need
    ESD protection, but that could still be achieved. Has anyone
    experimented with a 2-wire custom physical layer with a high
    differential input impedance?

    Thanks in advance.
     
  2. PeteS

    PeteS Guest

    I don't think you are going to achieve 10M of differential impedance
    with 4000 devices hanging off the bus. It may be possible, but with
    ordinary devices I don't really see how (I'm open to being convinced,
    of course :)

    Apart from all the wire capacitance, there's the pin capacitive loading
    of the devices. At a 1k data rate (500Hz line frequency), you would
    need to keep the total capacitance below about 30pF to achieve 10M of
    *capacitive reactance*, let alone impedance - tough, if not impossible
    to do with so many physical devices.

    CAN devices have to do a tradeoff in their input structure (see the
    SN65HVD231 series datasheet from TI for a nice equivalent input
    circuit) to balance loading against speed. No-one will buy the devices
    if they aren't rated at least at 250kb/s (and most are rated at 1Mb/s).

    As with all such busses, data rate is inversely proportional to cable
    length, as your application obviously would be.

    Just my $0.02

    Cheers

    PeteS
     
  3. Mochuelo

    Mochuelo Guest

    I was talking about 10 Mohm per transceiver.
    I haven't found an ordinary device that exhibits that. That's why I
    was asking whether someone else either knows of a commercial device
    like that or has experimented with self-made devices.
    Wire capacitance should not be a problem at 1 kbps. Even if each one
    of the 4000 devices had the capacitance of an ordinary device, which
    is around 20 pF, that would make Ctot=80 nF, which, together with
    Rline=(60 ohm||2500 ohm)=58.59 ohm makes time_constant=4.7 us.
    Neglectable at 1 kbps, and so are reflections (max node-to-node
    distance is lower than 300 m).
    I would buy them, because I don't care about speed. I care about input
    impedance, and about satisfying dc thresholds.

    Best.
     
  4. Mochuelo

    Mochuelo Guest

    That was actually device differential input capacitance. Add some wire
    differential capacitance, and you still have a large margin, up to the
    1 ms time per bit.
     
  5. PeteS

    PeteS Guest

    On a per device basis, I don't think you'll find ultra high impedance
    inputs on devices designed for differential *signalling* (such as can,
    lvds, etc.,etc).

    What you may wish to look at is LIN bus transceivers. They were
    designed for lower speed applications (2.4kb/s - 20kb/s). A cursory
    search of TI shows the TPIC1021
    http://focus.ti.com/docs/prod/folders/print/tpic1021.html
    with input leakage on the data pin (LIN is a single wire system rather
    than differential) of +/- 5uA max. That's about 1.3M input resistance.
    Although not as good as the 10M you wish for, it's better than the
    differential transceivers you have looked at.

    A number of companies make LIN transceivers.

    Cheers

    PeteS
     
  6. Guest

    Maybe I'm confused and haven't slept much but aren't most differential
    signaling methods relatively low in differential impedance. i.e. by way
    of relatively small termination resistors. Off the top of my head LVDS
    in SATA is something like 150ohms (or is it 50?), ditto CAN, RS-485,
    etc. There is some signal integrity reason for this relating to
    preventing the RF waves from bouncing around.

    Accordingly, most differential signaling methods are low impedance
    almost by definition. I think what you're looking for is more along the
    lines of a single ended system. LIN has been mentioned in this thread,
    I don't know what the input impedance requirements of RS-232 are, CAN
    has a 1 wire version, ditto K-Line, LIN, etc. The problem becomes that
    high impedance systems are more prone to noise.

    What's your application? If you want a decent way to hook up many items
    to a network of some sort, your best bet may be to use Ethernet using
    some topology of hubs.

    Chris
     
  7. Mochuelo

    Mochuelo Guest

    I never said I would not load the line with 60 ohm (120 ohm on each
    side of the bus). I said I was looking for high-impedance
    transceivers. The total line termination impedance does not depend on
    the number of transceivers. The total transceiver impedance does.
    But the price of 4000 Ethernet transceivers is very high, and if I
    don't want to use coax cable, the price is even higher, since I need
    to place 4000 cables and use some large number of switches.

    All this is to interconnect 4000 sensors to be placed in a parking
    lot. Each sensor detects presence/absence of vehicle on its
    corresponding place, and informs a central computer.
     
  8. Uwe Bonnes

    Uwe Bonnes Guest

    Think about splitting up the bus...
     
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