The refresh pulses of ADuM3211

Hi, the ADuM3211 is employed in the isolation COMM of CAN and RS485.  Now it seems the isolator caused negative EMI issue with the GPS module nearby, when MCU is active but the COM line is unplugged. In datasheet(page 17) it mentions a periodic set of refresh pulses to ensure dc correctness, so I have questions about this mechanism.

1.What's the frequency and the duty cycle of the refresh pulses?

2.What's the meaning of the so-called "Unpower" in Table 35? Is it a real low-power state, or an assumption of the non-functoinality?

3.Can I disable the refresh pulses by disabling the COM port of MCU and unplugging the COM line at the same time?

Many thanks!

  • +1
    •  Analog Employees 
    on May 3, 2021 2:38 PM

    Hi Andrew, 

    The ADuM3211 is an edge encoding architecture. The isolator sends either a single or double pulse when the input detects a state transition. If the input does not detect a change within ~1us, it will send a refresh pulse. The refresh is for DC correctness, in case the output was somehow upset. The refresh pulses will be sent continuously with a DC input state, and they cannot be disabled.  

    If you have access to a scope, a probe can be placed between the bypass cap and the VDDx pin. If you set the scope with a DC offset, and look at the signal in one of the smaller mV ranges, you should be able to see the current pulses when the transmitter is active. 

    "Unpowered" means the supply is below the valid supply range. I recommend bring the supply to less than 0.5V to consider the device unpowered.  

    There are newer On-Off Keying architecture isolators available with the same pin-out. See the ADuM121N for the ADuM3211. These devices encode the state at the input, and do not have a refresh pulse. When isolator transmits in the non-default state, and effectively does nothing in the default state.  

    The ADuM3211 is a default low isolator. This means when the input side is unpowered, the output channel will go low. CAN transceivers transmit a dominant state when TXD is low. I would advice switching to a default high (ADuM3201 or ADuM121N1) to avoid CAN bus interruptions in case the logic side becomes unpowered. 

    Regards,

    Jason