ADuM7440 EMI vs Data-rate

Hi,

I'm looking at using the ADuM7440 in a design that has restricted space and I will therefore have a limited ability to build stitching capacitance inside the PCB (I only have space for maybe 5pF - 30pF depending on how I change the stack-up). I have some concerns about EMI so I have some questions about how the emissions from this device are affected by the input data rate. I had a look at AN-1109 and Table 4 and Table 6 in this document show quite a large reduction in emissions between a data rate of 10Mbps and 1Mbps. My questions are:

  1. How does this scale down to even slower data rates? For example, if I were to have an input data rate of 100kbps, would the emissions be significantly lower than those shown in Table 4 of AN-1109 (1Mbps)? I would expect so but I ask because of the next question:
  2. I see that there are a number of different models for the ADuM7440 (and ADuM1402) capable of various maximum data rates and I was wondering whether these devices all have the same EMI characteristics or do they perhaps have different rise/fall times for the encoding pulses? Or some other difference that affects the EMI characteristics?


Any information would be appreciated.

Kind regards,

Lionel

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    •  Analog Employees 
    on Apr 10, 2014 8:03 PM

    Hello Lionel,

    The ADuM7440 contains an internal refresh circuit that updates the output if the input stops switching or slows down below ~1.2 Mbps. Becase of this, the emissions will be approximately constant below 1.2 Mbps. The ADuM1400 would exhibit similar levels.

    ADI recently released a new product, the ADuM1440, which is specified to 2 Mbps and is also available in the small QSOP. This part draw much less current and therefore exhibits lower emissions. The refresh runs at a lower frequency, so you would see improvements when moving to a lower operating rate. You can also disable refresh, reducing power/emissions even further.

    DaveC

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  • 0
    •  Analog Employees 
    on Apr 10, 2014 8:03 PM

    Hello Lionel,

    The ADuM7440 contains an internal refresh circuit that updates the output if the input stops switching or slows down below ~1.2 Mbps. Becase of this, the emissions will be approximately constant below 1.2 Mbps. The ADuM1400 would exhibit similar levels.

    ADI recently released a new product, the ADuM1440, which is specified to 2 Mbps and is also available in the small QSOP. This part draw much less current and therefore exhibits lower emissions. The refresh runs at a lower frequency, so you would see improvements when moving to a lower operating rate. You can also disable refresh, reducing power/emissions even further.

    DaveC

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