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ADUM2402 emission

I am using an ADuM2402ARW on a circuit.

We are experiencing an intermittent emission at ~127MHz. While probing the PCB with a near field magnetic probe, I found a peak similar to the one on the emission graph when the probe was over the ADuM2402. Unfortunately I have not been able to reproduce it since.

So I am trying to figure out what could cause the phenomenon. While looking at the circuit, I noticed there are some long tracks connected to ADuM2402 outputs; these tracks run on inner layers of the PCB so the capacitive load may be high. After a research I found a topic where the maximum load capacitance of ADuM2402 is 15pF. The data rate itself is low, 19200 bps, but during the transitions the output driver would have to cope with the load capacitance. So, it occurred to me there could some instability caused by this load as termination (e.g. a series resistor) was overlooked in this design.

The PCB layout itself closely follows the directions of the data sheet, with a short path from the 0.1uF decoupling capacitor between pins 1-2 and 16-15 and short path betweent pins 8 and 9 to 1 and 2 through an adjacent GND layer.

So, is there any chance the capacitive load could make the ADuM2402 to oscillate at this ~127MHz frequency?

  • Hi EVC,

    Thanks for your query.

    The  ADUM2402 has high slew rate and can radiate when connected to long traces. The series R and the CL 15pF can be used to form an RC filter which will filter out the higher frequencies and reduce the slew rate.

    Can you hack your circuit and test this theory by inserting a series resistor on the channels output?

    Best Regards,


  • Hello, Shane.

    Yes, luckily there are resistors close to the input and output to select between two physical UARTs, so I replaced the 0R resistors with 100R ones (lowest value I had at hand). I will do a EMC preview tomorrow. Fingers crossed. :)

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