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TIDA-00176: Sin/Cos over unshielded twisted pair maximum length.

Part Number: TIDA-00176

What is the maximum length of the unshielded twisted pair that can carry Sin/Cos signals without EMI/EMC issues?

  • Dear Jagadish,

    Thank you for your interest in TIDA-00176.

    In industrial drives we typically only see use of shielded encoder cables and therefore we only designed and tested this reference design for use with shielded encoder cables. We did not test with unshielded cables.

    We’ve designed the TIDA-00176 to for example to accept input Sin/Cos voltages with nominal 1Vpp down to 0.3Vpp, as well as assume a cable with a 120 ohm impedance.  Whether a specific shielded cable (wire gauge and impedance) at a specified cable length including the installed Sin/Cos encoder meets this condition can only be calculated by the person who knows this specific setup.

    I assume with EMC/EMI issues you refer to the IEC 61800-3 EMC immunity standard. We’ve designed TIDA-00176 to pass the IEC 61800-3 EMC immunity tests for ESD (IEC 61000-4-2), EFT (IEC 61000-4-4) and surge (IEC 61000-4-5) as per the IEC standard specified test setup, which includes minimum cable length too, as outline in the test section on the TIDA-00176 design guide. For example surge was tested with a 30m cable length, the IEC standard requires at least a 20m cable. We’ve tested data integrity in normal operating conditions up to 71m based on the specified encoder cable and Sin/Cos encoder, without the presence of any transient overvoltage.

    To pass the EMC immunity per IEC 61800-3 with an unshielded cable, the TIDA-00176 may need additional TVS diodes at its inputs, since the IEC 61000-4-5 surge test setup differs between use of  shielded versus unshielded cables. With a shielded cable the surge is applied to the cable shield at the encoder connector on TIDA-00176 with a 2ohm source impedance of the surge generator. With an unshielded cable the surge is coupled into cable wire versus Earth with a total source impedance of 40Ohm (for 4 wires e.g. 160ohm per wire) + 2ohm in the surge generator.

    However as indicated even with a shielded cable the maximum cable length is impacted by the specific encoder cable and the selected encoder. Therefore it is unfortunately not possible to even provide a generic comment on the maximum cable length without knowing (and testing) the encoder cable and encoder to be used.

    Regards,
    Martin Staebler