Reactances
Digital Designers need to be aware of following four reactances sources that a signal sees on the board- Normal capacitance
- Normal inductance
- Mutual Capacitance
- Mutual Inductance
Normal Capacitance
- The capacitance can be defined as the reluctance offered by the capacitor either to charge quickly or decay quickly.
- The initial current into the uncharged capacitor will be high, at short time scale, capacitor acts like short
Normal Inductance
- Inductance is everywhere, in all electric circuits
- The inductance can be defined as the reluctance offered by the inductor either to build up quickly or decay quickly.
- At short time scales, inductor acts as open circuit
Mutual capacitance
- Mutual capacitance is present wherever there are two or more circuits
- There will be interaction between two circuits. The level of interaction decays expoential with the increase in distance
- Unit: farads or amp-sec/volt
- Mutual capacitance coupling is like a parasitic capacitor connected between circuit 1 and circuit 2
- A mutual capacitance injects a current into circuit 2 which is proportional to the rate of change of voltage in circuit 1.
- Mutual capacitance is less harmful than mutual inductance
- Mutual capacitance of 8-10% may induce voltage levels of around 0.5 V
Mutual inductance
- Mutual inductance exists wherever there are two or more loop currents
- Coefficient of mutual inductance decreases as distance between two loop currents increases
- Unit:henries or volt-sec/amp
- Quick changes in current in loop 1 induces large voltage in loop 2
- Similar to mutual capacitance, the mutual inductance induces unwanted crosstalk between circuits
- Faraday's law states that the induced voltage Y(t) is proportional to the rate of change of flux in loop 1. Hence, a fraction of the total flux from loop 1 passes through loop 2.
- Overall, voltage induced in loop 2 is directly proportional to current in loop 1. Since magnetic field is vector quantity, the voltage induced reverses as direction of current reverses
