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Author Topic: Power Supplies  (Read 2637 times)

jfrigo

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Power Supplies
« on: October 02, 2004, 11:25:23 am »

Dan,

I've already received some excellent advice from Dave Collins on this subject, but I'm interested to hear your views as well. What are the things to be most aware of with power supplies for ADC and DAC? How much capacity (it depends on the circuit of course, but please discuss some of the things to keep in mind)? Shielding? Is it best to keep the power supply separate or can you put it in the same box as the converter with some reasonable shielding? Any differences in how careful you have to be for the digital side vs. the analog side? What else?

Thank you.
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danlavry

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Re: Power Supplies
« Reply #1 on: October 02, 2004, 06:01:55 pm »

“Dan,
I've already received some excellent advice from Dave Collins on this subject, but I'm interested to hear your views as well. What are the things to be most aware of with power supplies for ADC and DAC? How much capacity (it depends on the circuit of course, but please discuss some of the things to keep in mind)? Shielding? Is it best to keep the power supply separate or can you put it in the same box as the converter with some reasonable shielding? Any differences in how careful you have to be for the digital side vs. the analog side? What else?
Thank you.”


The best way to approach the design of a power supply is to first understand the specific requirements and weaknesses of the circuit you are dealing with, be it a converter an amp or what not. Let’s take an AD for example. I think it unwise to go and design the AD first, than see what kind of supply is need. It is best done side by side, to avoid surprises.

Here is an example for approaching conducted noise on the DC power lines.

So say I am designing an AD, and it has a +/-15V rails for analog, +5VA for analog logic section, and +5V digital. Next you figure out what is the power supply rejection of each circuit. To simplify, say you are looking at some circuit with 60dB power supply rejection – for each 1V supply noise 1mV “sneaks” into the signal path…
Say you know that 10uV disturbance is fine. The same 60dB ratio allows you a 10mV supply noise. You could use a noisier supply say 100mV, but have to improve the circuit rejection to 80dB, or do something else….

My point is that it is wiser to know the circuit you are dealing with. The same thing applies for shielding. You can go for lower value impedances, differential circuits that cancel EMI pickup and so on.

How much capacitance? Again, tell me how large and how fast a current step to expect, and again, what is the circuit rejection to supply change, and the answer will “fall in place”…

So any pre made list or advice is likely to be overkill, under kill or both…

BR
Dan Lavry
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