This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Differential standard for 100 meters 2 MHz communication

Other Parts Discussed in Thread: SN65LVDS047, SN65LVDT348, AM26LV32E, AM26LV31E

Hi all.

I have to connect a remote device to an existing board. The interface includes:

-          a 4-wire SPI port (SCK, MISO, MOSI, SS), where the board is master and the device is slave; the SPI maximum rate is 2 MHz: at the moment I am using it at lower rates (500 kHz), but it could be good to have room for higher speeds

-          a “loopback” SPI SCK signal, carried back from the device in order to take into account all the delays

-          a 1 MHz clock signal (generated on the board and used by the device)

-          an interrupt line generated by the device

-          power and ground lines.

The main problems are:

-          I have only a 2.5V source on the board

-          I  have to reach a distance up to 100 meters

-          I have to transfer power, so I need the lowest power solution available in order to keep the voltage drop across cables as small as possible.

The device contains only a digital accelerometer which draws < 1 mA and can be powered up to 3.6V. Moreover, the device should be as small as possible.

I was thinking about using LVDS, but I don’t know if it is a good idea for such a distance. Using SN65LVDS047 and SN65LVDT348 (4 drivers and 4 receivers), which I found to be the lowest power ones,  on the device side, I have almost 50 mA at 3.3V. With an AWG26 cable I have 13.4ohm x 50 mA x2 = 1.34 V drop, which would prevent all devices to work (provided that a 3.3V was generated in a way from the 2.5V source). Sure, I could use a lower gauge wire, but that would raise the cable dimension and weight…

Moreover, should I isolate all signals? The whole system will be used near a railway, so I’m afraid I could have interferences.

Any kind of suggestion will be welcome.



  • Greetings -

    It appears you have a need four signals to go 100m at a 500k to 2Mhz rate in  a rugged enviroment.  LVDS is generally more for higher speeds and shorter distances and it provides only a +/-1V common mode tolerance.  If this is DC coupled, I suspect RS422 is a better solution due to the wider common mode range and also larger signal swing.  The conservative how far / how fast curves indicate 1Mbps / 100m is in the range.  I would recommed to look at the 3.3V RS422 line drivers and receivers and deremine if that can meet the power requirments, and perhaps a 2.5V to 3.3V boost can be used.  If the enviroment requires more isolation, then that could be considered as well.

    John Goldie
    DPS APPS / SVA /


  • Hi John,

    thanks for your answer.

    Yes, I have some signals (7 signals: SCK, MOSI, SS, CLOCK going from master to slave and MISO, SCK_BACK, DATA_READY going from slave to master) to go 100 m at a maximum rate of 2 MHz in a rugged environment.

    After I had read your answer, I found this document by Thomas Kugelstadt on Analog Application Journal (4Q 2011): Extending the SPI bus for long-distance communication. This looks very close to what I need, and suggests some solutions similar to those you suggested.

    I also found AM26LV31E and AM26LV32E (quad driver and quad receiver) which I think could be fine for my application.

    On the driver data sheet I see that the typical voltage drop on a 100 ohm resistor is 2.6V, which means 26 mA. Considering all the seven drivers (both on master and on slave) I have 26 x 7 = 182 mA, which have to be added to the 8 mA typically drawn by the receivers, for a total of 190 mA (let's assume 200 mA). This means, with a 100 meter AWG26 cable (13.4 ohm), a voltage drop of 5.36 V, which means that even if I assume a 5 V boost for the power line I cannot do the job. With an AWG24 cable the voltage drop goes down to 200 mA x 8.42 ohm x2 = 3.37 V, which is still too much. To go under 1.7 V drop, I should use AWG20 or similar, but they are far too big to use in a similar application...

    Am I missing something? Generally speaking, the RS422 idea seems more robust than LVDS, but how can I solve this power issue? Are there any different solutions I could look at?

    Moreover, the document mentioned above suggests to use isolators, which I cannot put on the slave board (can I put them on the master board, just in case...?), and mean an extra power needed for the whole system...

    Thanks again,


  • Stefano

    As John mentioned, most of our high speed interface solutions look at higher data rates than what you need.   Is it a possibility to use a boost regulator to generate a local power supply at greater than your 2.5V rail, and then use something like RS-485 which falls nicely into your speed requirements?




    for more info on RS-485.


    Mark Sauerwald


  • Mark,

    I can surely use a boost supply in order to go higher than 2.5V (3.3V or maybe 5V), but with RS485/RS422, as I previously wrote, the problem moves to the power supply lines. If there's a way to solve the power issue, I can go for RS422 with the two devices I mentioned in my previous post, but I was looking for LVDS more for its low-power features than for its speed.



  • Stefano

    You had expressed your desire for low power in terms of avoiding voltage drop along the cable - the voltage drop is proportional to the current, not the power, so boosting to a high voltage, and then regulating down actually helps you a lot - as an example, if the cable represented 10Ohms of resistance, and you needed 1W at your load, then with 2.5V, you would need to send 400mA and would have huge loss along the cable - but if you were to transfer your power at 25V, then you only need 40mA, and power lost along the cable is a factor of 100 lower.


  • Hi Mark,

    to be honest I did not think about boosting voltage that high... I will give a look at the available boost devices and do some math and then I will come back here again to write down what I found... and get a little more help from you :)