Microcontroller - Using a inductor between AVDD & VDD

Bob Lawrence (VE1RLL)

I know that generally speaking that Inductors can smooth out the power to a device. I was wondering how the inductor improves the ADC noise rejection and if they mean internal ADC , external or both?


From here: [img][/img]

https://predictabledesigns.com/the-beginners-guide-to-designing-with-the-dspic33-microcontroller/

cheezus

In this example the inductor is meant to filter the analog supply voltage, presumably removing high frequency components of the digital circuitry. It's intended to improve noise performance of the INTERNAL ADC. The inductor basically blocks high frequency components from the digital section passing to the chip's analog supply section.


Rather than try to explain with my poor understanding ;


https://en.wikipedia.org/wiki/Inductor

Tubular

Yes, adding a second frequency dependent component results in a filter that has a sharper rolloff, so higher frequency components are attenuated more than a capacitor alone

The next step after L-C, is C-L-C (CLC) or 'Pi' filter, which has 3 components

Phil Pilgrim (PhiPi)

But an inductor alone won't be much help if it's connected between Vdd and AVdd next to the device. Better to run separate traces from the power source, and place the inductor in line with AVdd.

-Phil

tonyp12

Inductor does improve the ADC noise rejection.

AVdd needs to be of known and stable voltage, so the more you can choke it down but still meet the required uA the better.

I seen 1 Ohm series resistor and 10uF+0.01uF cap added too.

Mark_T

If the analog ground and digital ground are pinned out separately you can do something to isolate the grounds,
star-grounding under the chip (commonly mentioned in datasheets for ADCs and DACs), means ensuring the
digital supply current paths are kept away from the analog ground plane. Switching transients in that current
induce noticable voltages across stray inductance (even on a ground plane).

I tend to layout boards with a single groundplane of two parts linked by a single bridge. If that bridge is
an inductor it can help isolate voltage noise from the analog side, but even without that you isolate the
analog side from current noise in the groundplane.

If a chip has a single shared analog/digital ground pin you cannot eliminate the induced voltages across the
stray inductance of the ground bond-wire inside the package, so internal current switching noise is inevitably
injected into the analog side, even if the supply is extremely well decoupled on the PCB.

This is why its a known practice on a microcontroller to halt the processor while running the inbuilt ADC, relying on
the conversion-done interrupt to wake up the digital side of the chip.