Decoupling capacitor function is different for digital and analog circuits.
Digital circuits draw large currents when they are switched (all types), and if these currents must pass through the power distribution system they induce voltage spikes throughout the PCB causing noise and unreliable operation. Placing a decoupling capacitor (.1UF capacitors are small and inexpensive) across the power pins of each digital IC provides a local return path for the current spike thus preventing noise.
Analog circuits like op amps seldom generate noise because the currents are much lower, but they are very suceptable to noise generated by other circuits on the PCB. Placing a small resistor (20 to 51 ohms) in series with each power lead with a .1UF capacitor from the IC pin to ground creates a filter that shunts the noise to ground before it can reach the IC power pins. Check power supply rejection for op amps and you will see that it is quite low and it decreases with increasing frequency.
Decoupling is required to obtain a reliable low noise system.
Digital circuits draw large currents when they are switched (all types), and if these currents must pass through the power distribution system they induce voltage spikes throughout the PCB causing noise and unreliable operation. Placing a decoupling capacitor (.1UF capacitors are small and inexpensive) across the power pins of each digital IC provides a local return path for the current spike thus preventing noise.
Analog circuits like op amps seldom generate noise because the currents are much lower, but they are very suceptable to noise generated by other circuits on the PCB. Placing a small resistor (20 to 51 ohms) in series with each power lead with a .1UF capacitor from the IC pin to ground creates a filter that shunts the noise to ground before it can reach the IC power pins. Check power supply rejection for op amps and you will see that it is quite low and it decreases with increasing frequency.
Decoupling is required to obtain a reliable low noise system.