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LMX2492: Op-amp selection for active loop filter

AY
Prodigy 180 points
Part Number: LMX2492
Other Parts Discussed in Thread: PLLATINUMSIM-SW

Hi,

According to the data sheet, the charge pump leakage current is 10nA (max).

When selecting the op-amp for an active loop filter (eg. active type A), is the input bias current an important parameter to be considered? as some of the op-amps have input bias current in the order of micro amperes.

Thanks.

  • 0
    TI__Mastermind 27581 points
    AY,

    Input bias currents can lead to high spurs. In the old days with integer PLLs, it was a big deal, but newer PLLs have much higher phase detector rates and can tolderate much higher bias currents, althouogh uA range is still something to consider. That being said if you have a 50 MHz phase detector frequency, then 10 nA, 100 nA, or even 1 uA of bias currents likely will produce no noticable difference. The issue I was facing was it was hard to find something with good noise, low negative input rail, and higher voltage and still have low bias currents.

    For active filters, you might find the chapter in my PLL book (ti.com\tool\pll_book) on active filters useful as well as the PLLatinum Sim tool (ti.com\tool\PLLATINUMSIM-SW) as this models the active A filter and you can also model the impact of this leakage current on spur and even lock time.

    Regards,
    Dean