ADUM3402 and ADM3078 versus ADM2682

I have an existing RS485 half duplex design using the ADUM3402, the ADM3078 running at 3.3V and a DC-DC convertor. I'm looking to save some space (as always) and am considering using the ADM2682. However, I've been trying to interpret the supply current specifications, and however I look at it, the single device solution apprears to require about twice the current under the same conditions (60R load). There doesn't appear to be a spec. for the power conversion efficiency, so am I misinterpreting the specs? I need a low-power solution.

On the subject of the ADM3078, are there any known issues with the fail-safe detection features? I appear to get false start characters from the receiver on a low-capacitance bus (<600pF approx, 2 x 120R terminators) when the the transmitter turns off, with a baud rate of 240k. Once the bus capacitance rises above about 600pF, the voltage collapse appears to be slow enough for this not to happen.

  • 0
    •  Analog Employees 
    on Oct 24, 2013 9:40 AM

    Hi NTipping,

    What DC-DC convertor are you using? Is it an isoPower solution, such as the ADuM5000?

    The power conversion efficiency is not specified for the ADM2682E, it is restricted by the small size of the internal transformer and this is the reason for the high supply current.

    In order to achieve maximum integration (lowest board space), but a lower power solution than the ADM2682E, i can suggest using the ADM2485. The ADM2485 has integrated an RS-485 transceiver, iCoupler signal isolation, and a transformer driver into a single package. The ADM2485 will operate at lower power than the ADM2682E.

    Please see the ADM2485 datasheet pp16 Figure 34, which shows an applications diagram.

    For your second question can you please provide some more information?

    Are you refering to a Transmitter and Receiver on the same ADM3078 transceiver? Or on seperate bus nodes?

    Is there a Pull-Up resistor on the RO?

    Regards,

    Richard Anslow

  • Hi Richard,

    I'm using a discrete DC-DC converter - a TRACO 1W unregulated device (5V to 5V then post-regulated to 3V3). I've now looked at the ADM2485 and although the power consumption is similar to the discrete solution, the increased footprint of the external transformer plus rectification versus a dc-dc convertor pushes the real estate requirements up to about the same too. Ah well, I guess you can't have everything.

    I am referring to the transmitter/receiver on the same ADM3078, and there is a pull up on RO (although it is 100k).

    Best Regards,

    Nick Tipping

  • 0
    •  Analog Employees 
    on Oct 25, 2013 10:43 AM

    Hi Nick,

    If you remove the 100K pull-up resistor on the RO pin, then this will remove your false start/glitch issue on the receiver output for when the transmitter turns off. The RO pin will stay Fail-safe HIGH.

    Regards,

    Richard Anslow.

  • Hi Richard,

    Really? IF A/Y finish > B/Z  at the end of a character, RO is fail-safe high, and is pulled high with a resistor, why would it glitch low when the Transmitter is disabled if a pull up is present (A/Y-B/Z should never be < -50mV)? The Receiver is disabled during transmission, so I need a pull-up to keep RO high.

    Best Regards,

    Nick Tipping

  • 0
    •  Analog Employees 
    on Nov 4, 2013 11:33 AM

    Hi Nick,

    Can you please share your schematic for the ADM3078E?

    Can you please provide an oscilloscope plot showing the glitch issue that you are seeing for the RO pin? Can you please also show the DE pin, and /RE pin on the oscilloscope plot?

    Can you then please temporarily remove the pull-up resistor on the RO pin and repeat the oscilloscope plots?

    Regards,

    Richard