Simulation error in LTspice for RMS-to-DC Converter AD8436 and AD736

Hi,

As I followed the application note and the datasheet to test the real rms-to-dc converter circuit, I first chose the AD8436 because of its high operating frequency. However the simulation in LTspice always come the similar errors no matter which circuit I use. For example, I use the AD8436 circuit shown in the datasheet as below:

And here is my simulation circuit and error comes along:

This is not the only circuit I tried with AD8436, but also the circuit in the application note. In addition to AD736, but the same error comes along, really don't know why it keeps saying the floating connection. Hope ot hear from you. Thanks a lot.

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    •  Analog Employees 
    on Nov 5, 2020 9:28 AM 4 months ago

    Hi FChen978,

    The floating nodes in the LTspice error corresponds to the Crest Factor Capacitor and op amp's input and supply rails within the AD8436 as shown in the functional block diagram:


    And one of the common practices with dealing with unused op amps is to connect them as a voltage follower and connect the positive input somewhere between the supply rails (https://www.analog.com/en/analog-dialogue/raqs/raq-issue-46.html ). I suggest you do the same for this simulation too since you need to power up both the amplifiers to avoid the floating node errors. Attached is the simulation circuit I used with basic configuration.

    AD8436_Basic_config.asc

    It successfully converged and here's the result:
    (Vin = 2Vpk @1kHz)

    I hope this helps.
    Let me know if you have more concerns.


    And for the AD736, can you also show us the circuit you used?

    Best Regards,
    Dann

Reply
  • 0
    •  Analog Employees 
    on Nov 5, 2020 9:28 AM 4 months ago

    Hi FChen978,

    The floating nodes in the LTspice error corresponds to the Crest Factor Capacitor and op amp's input and supply rails within the AD8436 as shown in the functional block diagram:


    And one of the common practices with dealing with unused op amps is to connect them as a voltage follower and connect the positive input somewhere between the supply rails (https://www.analog.com/en/analog-dialogue/raqs/raq-issue-46.html ). I suggest you do the same for this simulation too since you need to power up both the amplifiers to avoid the floating node errors. Attached is the simulation circuit I used with basic configuration.

    AD8436_Basic_config.asc

    It successfully converged and here's the result:
    (Vin = 2Vpk @1kHz)

    I hope this helps.
    Let me know if you have more concerns.


    And for the AD736, can you also show us the circuit you used?

    Best Regards,
    Dann

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