Separate analog & digital supply MCU

[vindhyachal]

Posted on January 23, 2015 at 10:05

1. I am using STM32 - 32 bit cortex M3, it has separate analog & digital supply pins.

2. Datasheet says :

VDDA and VSSA must be connected to VDD and VSS, respectively.

3. Since I have 12 analog channels connected , so avoid any digital noise to come in analog signal can I connect a inductor = 10uH & cap = 0.1uF (as filter) between AVDD & DVDD.

4. Can I do so?

Edit: Or can I connect a separate 3.3V regulator for both VDD & AVDD with common ground.

But this I think won't work as datasheet saya they must be connected together

[AvaTar]

Posted on January 23, 2015 at 10:15

2. Datasheet says :

VDDA and VSSA must be connected to VDD and VSS, respectively.

IMHO that makes no sense. Why having then VDDA/VSSA in the first place ? The only requirement I can see is that VSS and VSSA must be connected, i.e. the same static potential. The respective ground planes, however, should be kept separate, according to the design rules (I'm no hardware designer/layouter).

[Tesla DeLorean]

Posted on January 23, 2015 at 18:03

The respective ground planes, however, should be kept separate, according to the design rules (I'm no hardware designer/layouter).

Well they'd (grounds) need to be bonded at some point. What you're trying to avoid is getting large CMOS switching currents, into the analogue supply. The electrons are going to take the most direct path, so think of the flow to and from the regulator(s), and try not to make your layout a maze with constrictions.The analogue supply runs the PLL/VCO and POR circuits.