CAN transceiver with high differential input impedance

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

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

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

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.

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.

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

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.

Think about splitting up the bus...

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Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de

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