ADA4961 output inductors

Hello

I have two questions about the output inductors for the ADA4961.

1) According to the ADA4961 datasheet, the recommended output inductors are Coilcraft 0805CS-471XJLC. The reason given in the datasheet is: "The self resonant frequency of these inductors is high enough so that it does not impact the performance of the ADA4961 at up to 4 GHz."

I had a look at the datasheet for these inductors and see that the typical SRF is given as 375MHz.

I must be missing something, but I don't understand how a SRF of 375MHz is "high enough" not to impact the performance at up to 4GHz.

If possible, please provide an explanation as to what the datasheet means regarding the SRF.

2) These inductors are physically quite large (0805 footprint). Coilcraft offer 470nH inductors in an 0402 footprint (see 0402DF-471XJRU). What inductor datasheet parameters should I be checking if I wanted to change to a smaller footprint inductor because of space constraints?

Thanks

Gavin

  • Hi, Gavint,

        The first self resonant frequency (SRF, where the inductance resonates with its parasitic parallel capacitance) for pull up chokes does not create any performance issue, as the inductor behaves like a very high impedance at this parallel resonance.  It can be used as an indicator for higher frequency resonance, which is often a series resonance and turn the inductor into a low impedance.

        The inductance needed depends on the lowest operating frequency needed (the lower 3dB cut off frequency due to a single inductor in parallel with a 50 ohm circuit is given by fc = R/(2.pi.L) = 17 MHz for 470 nH). If your min. signal frequency is higher, then you can use a smaller inductor value.  Physically smaller inductor are limited in the max. inductance and allowable DC current they can provide, and often have lower Q factors.  For pull-up inductors, any form factor that provides the inductance and supports DC current (~ 80 mA each side) you need will work.  Coilcraft 0402-AF-471 has a higher SRF and supports DC currents to 310 mA, so it would work for a very wide frequency range.

    Benjamin