24th June 2007

Things done:
Construct the op-amp circuit, with Vcc/2 circuit combined with the voltage amplifying part. However, the op-amp constructed only has 1 as gain instead of 2 the supposed value.

decoupling capacitor
Decoupling is the isolation of two circuits on a common line. It is ususally a low pass filter. It is used to prevent transmission of noise from one circuit to another. I am wondering whether this is the case to use for Analog circuit and Digital circuit. Decoupling usually consists of a capacitor together with a inductor.

Bypass capacitors help filter the electrical noise out of your circuits. They do this by removing the alternating currents caused by ripple voltage. Most digital circuits have at least a couple of bypass capacitors. A good rule of thumb is to add one bypass capacitor for every integrated circuit on your board. A good default value for a bypass cap is 0.1uF. Higher frequencies require lower valued capacitors. And bypassing capacitor should be as close as possible to the component which it needs to reduce noise for.

For my own design, I think most of the parts is only bypassing capacitor in between power and the ground. For the AGND and DGND which prof said the general practice is to connect a capacitor in between the meet point of AGND and DGND, I am wondering whether that is the purpose for bypassing or decoupling. Although I read some online documents also doing the same thing with just a capacitor. It seems that it is used as bypassing, but then why prof said the capacitor is used for preventing the noise from digital circuit to the analog circuit, which is the definition of the decoupling capacitor. Hope I can get some clarification or some book for reference.

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