I am designing a small microcontroller project (atmega). I plan to use 2 analog input channels, 2 digital input channels, and 2 digital output channels. I want the board to be powered from a 24VDC power supply.
My original idea was to use the LM2594 buck regulator to step down the 24V to 3.3V. However, I am worried that the output will be too noisy (since it is a switching regulator) for precise (10-bit) ADC measurements.
With adequate filtering on the output, can I get VCC to be stable enough for the Atmega? Have any of you done something like this? If so, what kind of filtering should I use?..pi filter? or just de-coupling caps?
I have also heard of people using an LDO (low dropout linear regulator) after the buck regulator to reduce the noise problem. Is this necessary?
“Adequate filtering” really means nothing until you specify how much noise your system can tolerate. However, in my experience I have seen more often than not a linear regulator (sometimes phrased precision voltage reference) used for the voltage reference to perform analog to digital conversions.
You might be OK with just some filtering on the analog power pins of the microcontroller. I typically just use a small SMD “ferrite bead” type inductor and capacitors. You’d want to do this anyway, since digital circuits will add noise of their own.
You also should use other features, such as a ground plane and segregating the sensitive analog section from the sources of noise.
After saying that, adding an LDO post-regulator can be an easy way of greatly reducing power supply ripple. Set your switchmode regulator a little above the LDO regulator voltage (you don’t want to waste too much power).
Yes I realize that my question cannot be definitively answered without the specific noise requirements. I was more just asking for general “best practice” advice in this situation.
I have read the APP notes from Atmel and Microchip that cover PCB layout concerns, so I have a pretty good handle on the benefits of the ground plane, decoupling caps, VCC - AVCC filtering, etc. However, I couldn’t find any information concerning output noise from on-board Switch Mode Power Supplies.
I did find one app note from National that has some really great layout information:
But I am still having trouble figuring out how much/what kind of filtering I will need on the output of the SMPS to provide a clean enough reference to my AVCC pin so that I can realize the benefit of full 10 bit resolution. (I realize there are many other factors other than the SMPS output, but lets assume I have addressed those and only need to worry about the noisy SMPS ouput.)