How to provide analog power

[Markus Zingg]

Hi all

This is most likely a dumb question, but

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?

TIA

Markus

 

[Tim Auton]

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
3.3V regulator to keep the digital noise at bay.


Tim

 

[Chris]

Hi all

This is most likely a dumb question, but

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?

TIA

Markus

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

http://www.analog.com/en/dcIndex.html

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.

Feel free to post again with more information.

Good luck
Chris

 

[Markus Zingg]

Hi Tim (and Chris)

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.

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?

Markus