About Passive Filtering (I)

Disadvantages of passive filters: Poor load capacity, no amplification, unsatisfactory edges are not steep, and each level affects each other.

RC filter:

1. C value selection: C can not be selected too small, otherwise the load capacitance has a great influence on the filter circuit. Generally, the input capacitance of the IC often has an input capacitance of l~lOpF. If the C value is too large, it will affect the high frequency characteristics of the filter circuit because the high frequency characteristics of the large capacitor are generally not good.

2. Selection of R value: If the R value is too small, the load of the power supply will be increased. If the R value is too large, more energy will be consumed.

The biggest drawback of the RC filter circuit is that it not only consumes the signal energy we wish to suppress, but also consumes the signal energy we wish to retain. In addition, due to the high frequency characteristics of the capacitor, it cannot be used in the case of too high frequency. For example, an LC filter is required for several MHz or more.

Capacitor filter circuit

When analyzing the working principle of the capacitor filter circuit, the direct isolation characteristics and energy storage characteristics of the capacitor are mainly used. The pulsating DC voltage outputted by the front rectifier circuit can be decomposed into a DC voltage and a set of alternating currents of different frequencies. The AC voltage portion flows from the capacitor to the ground, and the DC voltage portion cannot be due to the pass-through characteristics of the capacitor. Grounding flows to the next stage of the circuit. Thus, the capacitor removes the filtered portion of the original unidirectional pulsating DC voltage.

In addition, the capacitor filter circuit can also be explained by the capacitance storage characteristic. When the unidirectional pulsating DC voltage is at a high peak, the capacitor is charged, and when it is at a low peak voltage, it is discharged, so that the high peak voltage is stored to a low peak voltage. Release again. Converting the rugged one-way pulsating DC voltage into a relatively smooth DC voltage.

The capacity of the filter capacitor is usually large, and it is often the largest capacitor in the whole circuit. The filter capacitor has a large capacity and a good filtering effect. The capacitor filter circuit is the most commonly used one of various filter circuits.

How to choose the power filter capacitor, master the essence and method, in fact, it is not difficult.

1) Theoretically ideal capacitors whose impedance decreases with increasing frequency (1/jwc), but due to the inductive effect of the pins at both ends of the capacitor, the capacitor should be considered as an LC series resonant circuit, the self-resonant frequency The SFR parameter of the device, which means that when the frequency is greater than the SFR value, the capacitance becomes an inductance. If the capacitance is filtered to the ground, the suppression of interference is greatly reduced when the frequency exceeds the FSR, so a smaller capacitor is required to be connected in parallel. Can think about why? The reason is that the small capacitor, SFR value is large, provides a grounding path to the high-frequency signal, so we often understand this in the power supply filter circuit: large capacitance considers low frequency, small capacitance considers high frequency, fundamental The reason is that the SFR (self-resonant frequency) values are different. Of course, you can think about why? If you think from this point of view, you can understand why the capacitors in the power supply filter should be as close as possible to the ground.

2) So in the actual design, we often have doubts, how do I know the SFR of the capacitor? Even if I know the SFR value, how do I choose the capacitance value of different SFR values? Is it a capacitor or two capacitors? The SFR value of the capacitor is related to the capacitance value, and is related to the pin inductance of the capacitor. Therefore, the SFR value of the 0402, 0603, or in-line capacitor of the same capacitance value will not be the same. Of course, there are two ways to obtain the SFR value:

1) Device Data Sheet, such as 22pf0402 capacitor SFR value is about 2G;

2) directly measure the self-resonant frequency through the network analyzer, think about how to measure? After knowing the SFR value of the capacitor, use software simulation, such as RFsim99, select one or two circuits in the working frequency band of the circuit you are supplying. There is enough noise suppression ratio. After the simulation, it is the actual circuit test. For example, when debugging the receiving sensitivity of the mobile phone, the power supply filtering of the LNA is the key, and good power supply filtering can often improve several dB.

The essence of the capacitor is through AC and DC. In theory, the larger the capacitor for power supply filtering, the better. However, due to lead and PCB wiring, the capacitor is actually a parallel circuit of inductor and capacitor (and the resistance of the capacitor itself, sometimes not negligible). This introduces the concept of resonant frequency: ω = 1 / (LC) / 1 2 Capacitance is capacitive below the resonant frequency, and the capacitance above the resonant frequency is inductive. Therefore, the general capacitor filter low frequency wave, small capacitor filter high frequency wave.

This also explains why the capacitor filter frequency of the same value STM package is higher than the DIP package.