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LM15851EVM: power supply network (ferrite beads usage)

Part Number: LM15851EVM
Other Parts Discussed in Thread: LMK04828, TPS54327, LP3878, LMX2581, LM15851


I'm designing a power supply network for an RF-sampling wideband receiver and use the LM15851EVM as a starting point. As such a system requires well filtered low noise power supplies for the sensitive devices used, the ferrite beads are being used to clean up power rails. So, I have several questions concerning filtering strategy used in the LM15851EVM.

As an example, let's take one of the LMK04828 power supplies (say the Vcc2 net). It all begins from the TPS54327 switching regulator. If I understood correctly, the purpose of the low pass filter formed by L7, C158, C159 is to filter out the residual voltage ripple produced by the switching regulator. Also, in the most common approach, the output impedance of that filter seen by the load should be kept low enough and shouldn't have considerable peaking in all the used frequency range.

So, I decided to check this LPF with the Microwave Office (NI AWR Design Environment). I used the LPF components' models provided by the manufacturers. What I've seen is that the filter transfer function has ~17 dB peaking at ~70 kHz. Also I've seen that the output impedance of the filter has a peak at the same frequency and reaches ~3 Ohms.

The filters placed after the LP3878 LDOs and on the individual power pins of the LMK04828 also have peaks in the frequency region of hundreds of kHz. The LPF formed by the L2 and C103 (and similar LPFs) has the output impedance peak of ~100 Ohm at ~400 kHz.

So, on the one hand we have a requirement to eliminate resonant peaking and on the other hand we have peaking all around. I'm confused.

As I understand it, the most effective way to damp such peaks is to use large electrolytic capacitors with some ESR following ferrite beads. But the LMK04828 and LMX2581 have many different supply pins so the use of electrolytic capacitors for each such pin seems not reasonable.

So, I have the following questions.
Do I miss something? Is my approach too conservative? Which strategy did you use when were designing power supply network for the EVM?

Thanks in advance.

Best regards,

  • Dear Vic,

    You are quite right, power supply should present to the load a nice low Z to the load with a flat response in the spectrum of interest.

    I do not think there is anything worng with your approach. You likely found some weaknesses in the power supply of our LM15851 design.

    Also, LMX2581 and LMK04828 are devices with a large amount of on chip regulation so if the power suplly is not perfect, you will still get excellent performance.

    I would recommend to follow the guidelines of the EVM for each device for power supply decoupling.

    I hope this helps

    Regards, Simon.
  • Hi, Simon.

    Thank you for the answer.

    Regards, Vic.