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busypotato , 08-26-2026, 04:45 PM

Hello @robertferanec, @philslab, and everyone else,

I'm currently working through the Mixed Signal Hardware Design course on Fedevel, specifically Phil's Lab's course. I'm on Lesson 6, in the microcontroller circuitry section.

In this lesson, Phil covers the circuitry for VDD and VDDA (the digital and analog power pins). As shown in the screenshot, he uses the digital supply (+3.3V) and filters it to generate VDDA. However, in Lesson 5, we covered power supply circuitry and created a dedicated analog supply (+3.3VA).

I'm a bit confused about why we'd use the digital supply for analog circuitry when we already have a dedicated analog supply. The only explanation I can come up with is that the analog supply couldn't provide enough output current (but I am not sure why we simply didn't choose a more powerful LDO to begin with in this case), so I'm not sure if it is correct.

Thanks in advance for any clarification!

, 08-26-2026, 04:53 PM

🤖 In the meantime, here’s a response from the [FEDEVEL AI HW Assistant]().
It helps to find relevant resources for your question, but it may make mistakes as it’s still in early development.

This is the response from the AI Assistant:
The approach of using a filtered digital supply for analog circuitry (VDDA) while having a separate analog supply (+3.3VA) is typically driven by **noise management** and **system partitioning**. Here's the breakdown:

1. **Noise Isolation Requirements**:
- Digital supplies inherently have switching noise from logic transitions. For *high-precision analog circuits* (e.g., ADCs, sensors), a dedicated analog supply (+3.3VA) is critical to avoid noise coupling.
- However, **microcontroller analog pins (VDDA)** often have built-in noise rejection (e.g., PSRR - Power Supply Rejection Ratio). If the analog section isn't ultra-sensitive, a filtered digital supply may suffice. Ferrite beads/LC filters (as shown in Phil's design) attenuate high-frequency noise to acceptable levels ([Example: PSRR discussion](https://www.youtube.com/watch?v=LDe3ZJ16HxQ&t=2466)).

2. **Current and Cost Optimization**:
- Dedicated analog LDOs (+3.3VA) are ideal for noise-sensitive circuits but add cost/space. If the microcontroller's analog current draw is low (e.g., <100mA), filtering the digital supply avoids needing a larger/more expensive LDO ([Power domain considerations](https://www.youtube.com/watch?v=vnSX34YLjCo&t=2847)).

3. **Design Context**:
- Lesson 5’s +3.3VA might serve *critical analog components* (e.g., high-resolution ADCs), while Lesson 6’s VDDA could be for less sensitive microcontroller analog I/O.

, 08-26-2026, 04:53 PM

This partitioning balances performance and practicality.

**Key Takeaway**: Use dedicated analog supplies for high-precision circuits, but leverage filtered digital supplies where noise tolerance allows. Always check the microcontroller’s VDDA specs – many specify acceptable ripple levels that a well-filtered digital rail can meet.

You can continue chatting with the AI Assistant here:
https://fedevel.com/hw-assistant

Lukas , 08-26-2026, 06:22 PM

Ah yes, i remember looking at that as well whislt doing the course. I think I came to the conclusion that the analog voltage on the MCU and the one on the ADC converters don't require the same performance. Ultimately this MCU is not doing a whole lot, so there is no reason to hook up our clean analog reference to this noisy chip, so instead we just copy the digital reference voltage, throw a pi filter on it, and call it a day. I didn't go the distance of trying to quantify this performance difference, but that could be an interesting exercise

QDrives , 08-27-2026, 02:04 PM

So what gets supplied from the 'analog' LDO?

Lukas , 08-27-2026, 04:55 PM

According to AI, its the internal ADC converters of the MCU, which you are not using because you have external ones, and then some basic functionalities like bus voltage monitoring , tracking the internal battery levels and the internal temperature sensors, where your improved analog supply voltage will not really make the difference. So in short, you need to hook it up, but it wont make a big difference in this circuit

busypotato , 08-28-2026, 11:14 AM

Thank you, Lukas. Phil is saying in the lesson that we are not using MCU's ADC, so maybe he also did it just for the sake of another filter demo with an educational purpose