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TPS7A84: Noise Rejection Ratio of NR/SS pin to OUT

Guru 20090 points

Part Number: TPS7A84

Hello,

Are there any data of Noise Rejection Ratio of NR/SS pin to OUT? Or Could you please advice the measurement method of this?

The ceramic capacitor generate piezoelectric noise.
The capacitor at NR/SS pin generate piezoelectric noise, we can see the high frequency AC noise due to the ceramic capacitor at the output.
I would like to check how much the OUT pin is affected when noise is applied to the NR / SS pin.

I appreciate it if you would support this post.

Best Regards,
Ryuji

  • Hi Ryuji,

    If you inject a small signal into the NR/SS pin it will likely be a 1:1 correlation on the output. The NR/SS pin provides the reference to the error amplifier, which is used to regulate the output.

    Could you provide some scopeshots showing the capacitor generating piezoelectric noise? How are you measuring the noise? Are you using a spectrum analyzer or another method?

    Regards,
    Mark
  • Guru 20090 points

    In reply to Mark Sellers:

    Hello,

    I appreciate your reply. We measured the noise by spectrum analyzer.
    Could you please let me know the band width of TPS7A84 Error amp?
    Also, if we injected the noise to the NR/SS pin via AC coupling caps, are there any problem?
    I think that the soft start time may be change. I appreciate it if you have other idea to inject the noise to the NR/SS pin.

    Best Regards,
    Ryuji
  • Guru 20090 points

    In reply to Ryuji:

    Hello Mark san,

    I would be grateful if you could let us have your answer concerning this matter.

    Best Regards,
    Ryuji
  • In reply to Ryuji:

    Hi Ryuji-san,

    You should be able to inject noise to the NR/SS pin using an AC coupling cap, but what was the frequency you are seeing the piezoelectric noise at? Keep in mind this part does have a charge pump and there will be noise spurs at 4MHz and 8MHz as shown in the datasheet. It is very unlikely that the capacitor is generating measurable noise as we use ceramic capacitors for the noise plots shown in the datasheet.

    Best Regards,
    Mark
  • Guru 20090 points

    In reply to Mark Sellers:

    Hello Mark san,

    I appreciate your reply.
    Could you please let me know the bandwidth of error amp of TPS7A84?
    Also, I couldn't found charge pump noise information from the datasheet.
    I think that the capacitor generate the noise when we use air cooling also the boards is vibrated.

    Best Regards,
    Ryuji
  • In reply to Ryuji:

    Hi Ryuji-san,

    The loop bandwidth of the TPS7A84 is difficult to measure externally because there is an internal CFF capacitor that cannot be removed. If you try to measure the bandwidth of the LDO using a Network Analyzer it will measure ~150kHz. However, the actual bandwidth of the LDO is ~1MHz.

    You are correct that curve is not in that datasheet, if you look at the TPS7A84A datasheet it will show the charge pump spurs being generated by the LDO.

    It is possible if the board is vibrating that the capacitors generate noise. Would you be able to provide a plot of the noise you are measuring?

    Regards,
    Mark
  • Guru 20090 points

    In reply to Mark Sellers:

    Hello Mark san,

    I appreciate your support.
    I got this question from my customer and I don't have any plot now.
    I will inform if I got a plot which can be share.

    Best Regards,
    Ryuji
  • In reply to Ryuji:

    Hi Ryuji-san,

    If you get a plot from your customer please share it. For now I am going to close this thread. Let me know if there is anything else we can help you with.

    Regards,
    Mark

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