Generating separate 5V supply for Analog supply

[cjrathi]

Hi,

I am designing a board which requires 5V (digital), 5V (analog), 3.3V, 1.8V and 1.2V supplies. Initially my plan was to take 12V as an input and generate all these supplies from separate DC-DC switcher. The only reason for not taking 5V as an input is that I need separate 5V to be generated for analog and digital section, to reduce noise coupling between analog and digital section.

Now I am thinking of taking 5V as an input and use it for digital 5V section and generate other supplies from this 5V. But now issue is how can I generate 5V for analog supply.

Is there any way so that I can use separate supply for 5V analog section?
I tried searching on google and found out following option:-
1. Using voltage doubler circuit (5V-10V) and than use LDO to generate 5V analog supply
2. Using IC 555

But above options have current limitation of around 50-100mA. I may need around 750mA of current for 5V analog section (one 12-bit ADC and 4 dual output Op-Amps).

Can any one please suggest me something which will suffice my requirement?

Thanks in advance

 

[albbg]

You can use a 5.5 V as input voltage, then you can supply the digital and the analog sections by means of two separate 5V LDO. The other voltages can be derived from the 5.5 V using LDOs or switching units.

 

[cjrathi]

You can use a 5.5 V as input voltage, then you can supply the digital and the analog sections by means of two separate 5V LDO. The other voltages can be derived from the 5.5 V using LDOs or switching units.

Yes. I have that option, but 5.5V will be non-standard input. I want to power up the system with standard 5V adapter.

 

[KlausST]

Hi,

Why not using LC filters for analog and/or digital section? (My preferred solution)

Or you could use SEPIC or buck-boost converters.


750mA is a lot for an ADC and some OPAMPs. Is it that high speed, or why that much current?
On the other hand 12 bit is not very high resolution, so it is very likely that an LC filter is enough..

Klaus

 

[D.A.(Tony)Stewart]

in any good design , you define all Ouputs with, V, I and max ripple allowed. then additional filtering can be added as req'd

 

[Warpspeed]

It depends if you are planning to use a common ground for analog and digital, as noise problems often have far more to do with ground, than the supply rails which should be fairly easy to filter.

I would be looking at the overall ground plan first, if noise is anticipated.
Then work out how and where to power various sections.
Fully isolated dc supplies may be a forced requirement to isolate the dc returns from a reasonably clean reference equipotential ground .