Do you know WHY you’re supposed to use decoupling capacitors?
[Bertho] really enjoyed pawing through the pile of projects submitted to the 7400 logic contest. But one thing kept hitting him with the vast majority of the entries: decoupling capacitors were missing from the circuits. If you’ve worked with microcontrollers or digital logic chips you probably know that you’re supposed to add a small capacitor in between the voltage and ground pins for decoupling purposes. But do you know why? [Bertho] put together a great post that looks that the benefits of using decoupling capacitors in your circuits.
He set up a circuit using a 74HC04 inverter and put it to the test. The image above shows current measurments with the inverter under load. Images on the right show a decoupled circuit and the ones on the left shows a circuit without that capacitor. You can see that the decoupled circuit has much smoother signals when driven high. But it’s not just the smoothness that counts here. [Bertho] goes on to discuss the problem of slow rise-time caused by a dip in current flowing into a chip’s VCC pin. It can take a long time to get above the threshold where a chip would recognize a digital 1. Throwing a capacitor in there adds a little reservoir of current, just waiting to fill in when the power rail dips. This feeds the chip in times of need, keeping those logic transitions nice and snappy.
Filed under: Microcontrollers
[Bertho] really enjoyed pawing through the pile of projects submitted to the 7400 logic contest. But one thing kept hitting him with the vast majority of the entries: decoupling capacitors were missing from the circuits. If you’ve worked with microcontrollers or digital logic chips you probably know that you’re supposed to add a small capacitor in between the voltage and ground pins for decoupling purposes. But do you know why? [Bertho] put together a great post that looks that the benefits of using decoupling capacitors in your circuits.
He set up a circuit using a 74HC04 inverter and put it to the test. The image above shows current measurments with the inverter under load. Images on the right show a decoupled circuit and the ones on the left shows a circuit without that capacitor. You can see that the decoupled circuit has much smoother signals when driven high. But it’s not just the smoothness that counts here. [Bertho] goes on to discuss the problem of slow rise-time caused by a dip in current flowing into a chip’s VCC pin. It can take a long time to get above the threshold where a chip would recognize a digital 1. Throwing a capacitor in there adds a little reservoir of current, just waiting to fill in when the power rail dips. This feeds the chip in times of need, keeping those logic transitions nice and snappy.
Filed under: Microcontrollers