I do have a digital design requiereing 3.3 and 5v supply. One of the chips however is a mixed signal kind and requieres analog ground and analog power (also 3.3v). The external voltage I can use is 12v and apart from this mixed signal chip, I would have used two voltage regulators in series, one regulating down to 5 and from there to 3.3v. Overall power consumtion is < 500mW. Can I now simply use a third regulator to say regulate the 5v once more down to 3.3 and use it for the mixed signal analog supply pins? Is this the way this is usually done?
The purpose of the separate supply is to keep digital switching noise out of the analogue section. Voltage regulators alone generally aren't all that great at eliminiting this kind of noise, so just adding a 3.3V regulator to your 5V digital supply (which will have a good bit of digital noise) may not be the best solution.
How much effort you need to go to depends on how clean you need your analogue supply, which depends on the size of the analogue signals and the accuracy you need. Can you give us any idea? If there's an ADC involved, the number of bits of resolution would be a useful indicator.
Simply decoupling your existing 3.3V supply with an LC filter may be good enough, this is recommended in datasheets by microcontroller manufacturers and would be OK for, say, 3V signals and a 10-bit ADC. A series inductor from the 3.3V regulator to the analogue supply pin of your IC, and a capacitor between the analogue supply pin and ground.
If that's not enough (say, you have a 16-bit ADC), then a separate 3.3V supply with its own regulator would be useful. If your 12V supply is fairly clean (no PWMed motors, no switching regulators...), run the regulator from that. If the 12V is particularly noisy, run the 3.3V regulator from your 5V. Either way, use an LC filter on the input to the
Hi, Markus. Start with these Analog Devices appnotes of general interest:
AN-202: An IC Amplifier User's Guide to Decoupling, Grounding, and Making Things Go Right for a Change
AN-345: Grounding for Low-and-High-Frequency Circuits
AN-214: Ground Rules for High Speed Circuits
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Your question is kind of application specific, and everything depends on the details.
If you've got analog ground sliding through digital ground, with trace inductances (high speed) and trace resistances (precision DC voltages), this can cause all kinds of problems. Or not -- it depends entirely on you app and what you're trying to accomplish.
Thanks for your reply(s). The mixed signal part is acutally an image sensor (should have mentioned that in my first post) and the ADC seems to be 10 bit (at least that's what I get out of it on it's digital end) . So, the LC filter should be sufficient - right?
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