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AMC3330: AMC3301 AMC3302 high frequency output noise

Johann Stadler
Prodigy 160 points
Part Number: AMC3330

In the isolated amplifiers with integrated DC/DC converter there is some noise at the output pins OUTP and OUTN.
I understand that it is assumed that this noise is filtered out by a differential amplifier. But I am not so sure about the high frequency components.

Can you recommend some passive filter components, specifically for the high frequency components?

Regards

JS

  • 0
    TI__Mastermind 47625 points

    Hi JS,

    What is the frequency of the noise that you are seeing?

    The table below shows a few examples of output filters that work well with our AMC devices. Of course, the frequency of the signal being measure and what the output is being fed to is important to consider as well.

  • 0 in reply to Alexander Smith
    Prodigy 160 points

    Hi,

    thank you for your answer.

    I found that the table is from the Users Guide of a different part (AMC135x).

    On the oscilloscope with a passive probe I saw noise components with ~30 MHz and ~100 MHz. I do not have a picture at the moment.

    But I have a picture of a measurement with an active differential probe, see attached picture.
    Above: Time domain
    Below: FFT
    The bandwidth limit was set to 20 MHz at the oscilloscope.

    There are noise components at 10..20 MHz, 31 MHz, 62 MHz, 93 MHz,...

    Maybe we can try to increase the capacity of the differential capacitor.

    Will increasing the value of the series resistors also help?
    What about the capacitors against ground (C7 and C12 in the EVM)?

    Regards,
    JS

  • 0 in reply to Johann Stadler
    Guru 140200 points

    Hi Johann,

    is this being measured at the output of Alexander's differential RC low pass filter?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 160 points

    Well, this information was missing in the post.

    The picture in the previous post was taken with this filter:
    Series resistors 1k, Cs against ground 10nF (fg ~16 kHz),
    there is no differential capacitor.

    Here is another picture, setup same as above.
    The filter components:
    Series resistors 50 Ohm, Cs against ground 1 nF (fg ~3 MHz),
    again no differential capacitor.

    As expected, the noise at low frequencies (<2MHz) is much lower with the first filter.
    What is not clear for me, is, why the noise at higher frequencies (>10MHz) is higher with the first filter.

    Additional information: The measurements do not show the same TI device, but 2 devices on one board with different filter components.

    Regards,
    JS

  • 0 in reply to Johann Stadler
    TI__Mastermind 47625 points

    Hi JS,

    This document, specifically the introduction, goes into some details about the 30MHz noise from the integrated DC/DC converter. 

    https://www.ti.com/lit/an/sbaa515/sbaa515.pdf

    Can you please share details about your layout? 

    Given that there is almost always a ground plane on the low side, we've found that having minimal ground plane/trace lengths on the high side of the AMC33xx devices is best for minimizing the radiated emissions as it creates an asymmetric dipole antenna that does not radiate as effectively.

    What are the outputs feeding to? A differential amplifier as you mentioned in your OP?