Analog Digital voltage seperation in the board

[imman]

The analog section is more sensitive to noise due to the near by switching digital circuit. Most of the time I saw, inductor is provided to connect the analog supply voltage to an IC from the digital voltage. Why inductor is used? Is it for oppose the current change change due to the digital circuit on analog circuit.

What is the exact value of inductor to use? or how to calculate?

Thanks & Regards,
Imman

 

[kender]

Inductor together with all of the decoupling capacitors on the analog side forms an LC low-pass filter, which reduces the noise from the digital circuit. So, figure out the highest frequency of the noise that you want to reduce and calcualte the inductor value as for an LC filter. Or you could just pick the biggest inductor value that would fit your current rating, mechanical constraints and budget.

Another approach in addition to inductors is to use separate voltage regulators fro digital and analog. It's highly advisable to power the analog circuit from the linear regulator.

Yet another approach is to use ferrite beads instead of inductors. They work well at high frequencies (from tens of MHz to hundreds of MHz).

Also make sure that analog and digital and power circuits are star-grounded.

P.S. By the way, what kind of instrument are you building?

 

[imman]

Hi

Thank you for the inputs.
I am building modem device for FAX related application.

I saw in the referance design 4.7uH inductor is used to short the analog supply voltage with digital. (It is difficult to add a seperate regulator for analog power at this time). So I want to go with filter. I am not sure about the capacitence value due to decoupling in the board. So I have a doubt that, how they selected this 4.7uH ?

Regards,
Imman

 

[kender]

I am not sure about the capacitence value due to decoupling in the board.

For a relatively low frequency noise, the capacitance in the LC filter is at least the sum of all decoupling capacitors connected between the supply and ground.

 

[wonbef]

for high frequency, inductor has a large reactance.

 

[Kral]

imman,
To play it safe, use the value of the decoupling capacitor that is directly adjacent to the analog device. The rest of the decoupling capacitors are likely to have significant inductance in series, due to the inductance of the printed crcuit board.
Regards,
Kral