ADuCRF101 decoupling capacitors


Are there general requirements for the decoupling capacitors on IOVDD like there are for many other pins on the ADuCRF101 (see the datasheet)?

Does the ADuCRF101 really need 8 decoupling capacitors in parallel? Instead of having 4 0.1 uF caps and 4 100 pF caps in parallel, why not just have 1 0.4 uF and 400 pF cap in parallel? If my memory serves me right, there are different valued caps in parallel because there are different frequencies of noise to suppress? I am a little new to picking out the right decoupling parts, but that is the only explanation or reason I can even remember that makes slight sense.

  • 0
    •  Analog Employees 
    on Jul 20, 2011 5:11 PM

    A real capacitor has characteristics not only of capacitance but also of an inductance and resistance. It can be modeled as an ideal capacitor in series with an ideal inductor (ESL) in series with an ideal resistor (ESR). Capacitors series inductance (ESL) causes resonance at the Self Resonant Frequency (SRF). The ideal capacitor's impedance is infinite at DC and decreases as the applied voltage increases in frequency.

    In a circuit a capacitor acts as a low impedance element, only for a finite range of frequencies. Often smaller value capacitors are used in parallel with the main decoupling capacitor to provide a high frequency and a low frequency filtering effect, similar to what you see on the IOVDD line.

    The impedance of the capacitor can be calculated by

                                  Z=R + j(ωL– 1/ωc)

    ‘R’ being the real part and (ωL &1/ωc) the imaginary part. As the frequency moves away from resonance, one of the terms, inductive or capacitive, becomes much smaller than the other.

    Choose the capacitor value based on the self resonant characteristics defined by the manufacturer data sheet. Capacitors in parallel will help lower the total impedance characteristic.

    Decoupling capacitor usually need to be placed as close as possible to the pin to be effective and minimize additional series inductance. In the case of ADuCRF101, there are 5 supply pins requiring decoupling capacitors: IOVDD x 2, AVDD, and VDDBAT x2.

    Refer to ADuCRF101 mini board schematics (sheet 2) which shows 4 x IOVDD. This needs to be corrected to as referred above. Note when operating the mini board directly from a battery, for correct operation, a minimum of 68μF capacitor should be placed in parallel with the battery.