role of filter capacitor

[raviram87]

In any digital circuit, usually the Vdd - 5V and Vss = GND are given to the respective terminals in IC....

also a capacitor is usually connected from the power supply to the ground... why is this done?

recently i was working on a PIC based circuit... one of my mentors suggested that i tie my Vdd (5V) and the Vss (GND) together through a 0.1uF capacitor.... why is this done?

 

[Optikon]

In any digital circuit, usually the Vdd - 5V and Vss = GND are given to the respective terminals in IC....

also a capacitor is usually connected from the power supply to the ground... why is this done?

recently i was working on a PIC based circuit... one of my mentors suggested that i tie my Vdd (5V) and the Vss (GND) together through a 0.1uF capacitor.... why is this done?

Search for bypass capacitor. It is supposed to be a low impedance type capacitor(& connection) to provide a local supply of charge for when the IC demands a rapidy changing current. (like a high speed transistor switching & charging up internal node capacitance)

To do this correctly, make sure the capacitor has as short of a connection as possible to the pins of VSS & VDD. Make sure it is of the ceramic type. Otherwise, it will probably be ineffective at high speeds.

 

[raviram87]

suppose if i don not include such a capacitor in my design, will it affect the perfromance of the circuit in any way?

or can i run the circuit in the absence of these bypass capacitors?

 

[Optikon]

suppose if i don not include such a capacitor in my design, will it affect the perfromance of the circuit in any way?

or can i run the circuit in the absence of these bypass capacitors?

Probably OK but not a good practice.

 

[Torben]

I've had counter circuits go weird without the bypass caps in place. Everything seems to work fine for a while and then I get intermittent weirdness. It's happened more than once.

Since there is no reason not to use the bypass caps, now I just use them by default.


Torben

 

[Nigel Goodwin]

suppose if i don not include such a capacitor in my design, will it affect the perfromance of the circuit in any way?

or can i run the circuit in the absence of these bypass capacitors?

PIC's do seem fairly happy without them, but it's really an essential component - ALWAYS fit decoupling capacitors!.

 

[Papabravo]

There are two things going on with logic circuits.

1. Some chips demand more current from the supply when the outputs switch
2. Some chips are sensitive to high frequency noise on the VCC and GND

A large electolytic or tantalum, say 10 uF takes care of number one.
A small ceramic, say .01 uF takes care of number two.

People often ask why this combination is different that a single capacitor at 10.01uF. The answer is that total capacitance is only part of the story. The key is in the parallel branches and other features of the two types of capoacitor including but not limited to ESR and dielectric material. Most designers consider it cheap insurance to follow Nigel's suggestion.

 

[Diver300]

PICs seem to take more current when executing lines that change the program counters.

I had a problem with a PIC when the instructions executed were Return, Return and then GOTO.

I had missed out the capactior. The PIC behaved when I fitted it. As Torben said, always fit a decoupling capacitors. In high frequency or noisy circuits, fit lots of decoupling capacitors.

 

[Optikon]

Its ** ALL ** about impedance.. very little to do with actual capacitance. At high frequencies, you do not have the capacitance you think you do.. you must think about impedance to understand how to design properly.