Coupling capacitors
Since a capacitor tends to block the flow of DC current and allow the flow of AC current, they can be used to block DC currents from reaching test equipment measuring AC signals. Such leakage is called DC circuit loading (see What is the load of a circuit in DC Circuits). A capacitor in series with an AC test instrument is called a coupling capacitor. It will prevent DC loading of the instrument.A coupling capacitor is placed in series with the circuit being coupled to. This tends to form a high-pass filter. Therefore, you must choose a coupling capacitor that works with the circuit impedance such that desired frequencies are not rejected by the filter. You should choose a capacitor that creates a filter with a cutoff frequency equal to the lowest frequency you want to pass. However, that filter will introduce some phase distortion at lower frequencies. Therefore, you should choose a cutoff frequency that is about 1/10 of your desired cutoff frequency. In other words, choose a capacitor with a capacitive reactance (some people say impedance) that is 1/10 the impedance of the circuit at the desired cutoff frequency.
You can calculate the correct capacitor using a derivative of the formula used to calculate the cutoff frequency for an RC filter. Here's that formula.
| ƒCO
= |
1 |
| ——————— | |
| 2πRC |
Where:
C = Capacitance in Farads
ƒCO
= Desired cutoff frequency
R= Filter resistor
We can swap any term below the line for the term to the left of the equal sign (2π, R and C are each separate terms or they can be a single term together). We already know the cutoff frequency and want to know the capacitance. Let's swap those terms to create the formula to calculate the coupling capacitor. In this case, we have the circuit impedance instead of a filter resistor, so we also change the R to Z to represent the circuit impedance. In addition, we calculate the capacitor for a frequency that is 1/10 the desired cutoff frequency,[1] so we remove the CO subscript after the frequency.
| C
= |
1 |
| ——————— | |
| 2πZƒ |
Where:
C = Capacitance in Farads
ƒ
= 1/10 cutoff frequency
Z = Circuit impedance
Decoupling capacitors
Ideally, the wires supplying power to any circuit have pure unvarying DC. However, increasing current demand always causes the supply voltage to drop as current goes through the output impedance of the power supply. A regulated power supply mitigates this but may not be able to follow the rapid changes in current demand in digital circuits.. Even though the demand and changes are small, they cause corresponding changes in the supply voltage. This results in so-called AC noise or transient voltages. Longer wires or circuit board traces between the power supply and the circuit have more resistance. This results in even greater changes in supply voltage as current demand changes. A regulated power supply does not sense these remote changes and cannot mitigate them. The high-speed switching distributed around a digital circuit can cause significant noise on the power wires or traces that can affect nearby circuits. In nearby analog circuits this can result in noise in the output signal (such as audible of visible noise in audio and video circuits). Digital circuits can react to noise on the power supply connections as if it were an input signal, causing unpredictable outputs.
A decoupling capacitor is a filter capacitor that reduces AC noise from the power connections of a circuit. Decoupling capacitors are placed very close to the circuits so that there is little wiring between the circuit and the capacitor. This puts little resistance between the circuit and the capacitor, assuring maximum effect. They are common next to integrated circuits in digital circuits or mixed digital and analog circuits.
