How to achieve low impedance

7.5 How to achieve low impedance

We have seen in the examples that achieving low impedance between the power and ground rails is the key to the achieving noiseless power supply. Assume you have an IC that needs noise less power supply. If you have a impedance meter with you that can hook up between the power and ground pin and measure the impedance between the power and ground pins at all frequencies. If this impedance is low it is good. If this impedance is high it is bad. If this impedance is low, it means that the noise on power rail will get shorted with ground. If this impedance is high it means not all the noise on the power rail will short to the ground, and some of it will find its path to the ICs. How low should this impedance should be? We already calculated this in examples above. As a quick calculation you should at least take the current drawn by the IC as the delta I for calculation of required minimum impedance. If the IC has switching input output, it takes some current to charge the input output pins. The amount of the current drawn will be high if the load capacitance is high. The required impedance between the power and ground rails will have to be lower if there is the larger swing in the current.

We achieve this low impedance with the use of capacitors. As we have already seen in the capacitor chapter, the impedance of a capacitor is given by

Xc = 1/(2*pi*f*C)

where ,

f = frequency
C = Capacitance of the Capacitor,
Xc = Impedance of the Capacitor


The impedance is low if the capacitance is high enough. With this equation we can find the required value of the capacitor. We have already seen how to find the required value of the low impedance . If we just know what is a suitable value for the frequency our problem is solved.

We will consider two extreme cases of frequency. First consider that the frequency is very low. What does this frequency represent? This frequency represents the ripple frequency. We have seen that the change in the current drawn creates the ripple. And the rate of the change of the current determines the frequency. Since the current drawn changes randomly the noise generates is random. In case the frequency of the noise generated is very low, the drop across the power supply wiring inductor 2pfL is very low. If the frequency is very low, the drop across the power supply wiring inductor is so low that we do not need to worry about it at all. It is within the tolerance limit of many IC.

If the frequency of the noise generate is high, the drop across the power supply inductor 2pfL is high. The amplitude of the noise increases. The more the frequency, higher is the noise level. A capacitor is required. The capacitor shorts the high frequency noise.

The impedance of the capacitor also depends upon the frequency. Higher the frequency, lower the impedance. So if the frequency is arbitrarily high, say 200 MHz, the impedance of the capacitor will be very low by way of the capacitor impedance formula Xc = 1/(2*pi*f*C) . f the noise frequency is increased to 400 MHz, the impedance of the capacitor will be even lower. So we do not need to worry about high frequency. Unfortunately, we would have been happy if this were the case. Every capacitor has a parasitic inductance which ruins our dream of having a big capacitor to provide low impedance.

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