ESD on ADM2687 board

Hi All -

We are spinning a board which uses your very fine ADM2687 and are planning to add some degree of ESD protection.

Not trying to meet any particular standard at this time.

An MCC SM712 and 10 ohm series resistors as per AN-960 seems like a good idea.

I assume the SM712 should clamp to GND2, but am not sure how to handle the 485 shield.

Currently it just floats though one node grounds it (a RS485 dongle on the server).

We would like the possibility of having one of our nodes assume master status if the server fails or is absent, so it seems that a weak connection (1 -> 10K ?) between GND2 and shield may be desirable.

There is currently a 220V varistor between GND1 and GND2 though I am not sure what purpose it serves.

Should this be removed ?

Could you explain why terminating with a resistor and large, high quality, series cap is undesirable as mentioned in AN-960 ?

Also could you recommend a twisted pair cable with shield suitable for runs up to 1500 ft ?

Many thanks for any suggestions -


  • 0
    •  Analog Employees 
    on Nov 20, 2013 4:23 PM over 7 years ago

    Hi AlgaeEater,

    The ESD specification that you are trying to meet will dictate whether or not if you need external protection devices when using the  ADM2687E transceiver. The ADM2687E transceivers have ±15 kV ESD protection (HBM).
    ESD (Human Body Model) on A, B, Y, and Z pins.

    I can see from the MCC SM712 datasheet that this provides ~16Kv HBM.

    If you required HBM ESD protection, then the ADM2687E can be used, and you don't need the external MCC SM712 component.

    If you require IEC ESD protection, then i would recommend using external ESD protection devices (like a MCC SM712).

    AC termination is used to reduce the power consumption of idle links as well as to reduce ringing voltages.
    The negative effect though is a reduction in cable length and bit rate.
    The resistor-capacitor RC time-constant delay significantly reduces transmission speeds and cable distance.

    In electrically noisy environments, the shield can be used grounding only one end of the cable shield to prevent ground loop current flow.
    The one end of the cable shield that is grounded should be grounded through a low impedence.

    Category 5 cable STP (shielded twisted pair) is one of the most commonly used cables and is recommended.
    The cable has a maximum capacitance of 17 pF/ft (14.5 pF typical) and characteristic impedance of 100 ohms.

    Ground 1 and Ground 2 should not be connected together.

    Best Regards,

    Richard Anslow.