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LMX2594: Capacitors

Marvn Arcangel
Genius 16245 points

Part Number: LMX2594

Hi Experts,

In the typial application and recommended capacitor choice for the LMX2594, the datasheet cites two 0201 capacitors (refer to table 40). Can I clarify that the purpose of these capacitors is for a matching network? Furthermore, what are the requirements these capacitors need to fulfil? Must their SRF be far above the expected frequency of RFOUTAP and RFOUTAM?

Regards,

Marvin

  • 0
    TI__Guru** 119790 points

    Hi Marvn,

    The choice of these capacitors depends on the application. If the application needs 10MHz to 15GHz, the capacitors should behave like a short at these frequencies. The parts we specified in the datasheet can do so.

    If the application needs a single frequency or frequencies within similar range, then we don't need to use the datasheet specified capacitors, pick something else as long as they behaves as a short at the frequencies of interested.

  • 0 in reply to Noel Fung
    TI__Genius 16245 points

    Hi Noel,

    Just to be clear when you say "act like a short", you mean that the capacitor chosen should have a SRF as close to the frequencies of interest as possible? do I have to stick with the 0.01uF value? Or can I choose ANY capacitor as long as its SRF is near the freq of interest?
    If for instance I am using the LMX259 to output a 12.8GHz frequency, Would you happen to have any recommendations for such capacitors with 0201 footprint instead?
  • 0 in reply to Marvn Arcangel
    TI__Guru** 119790 points

    Hi Marvn,

    The capacitors are used to block the DC content but allow AC signal to pass through. In other words, they should behave like a 0Ω resistor to the AC signal. 

    Usually the capacitor vendors provide the characteristic data on their website, we can use the data to simulate the characteristic against frequencies. Pick the capacitor with the lowest impedance as possible. At 12.8GHz, usually it is just a few pF.