On the hardware page:
http://docs.spark.io/#/hardware/electrical-characteristics-power
It shows pin label “+3.3v* OUT”. which states " Filtered Output ".
Can Spark, elaborate on that please.
It’s just a filtered 3.3V node primarily for the Analog to Digital converter reference, and any sensors that will need to have a clean power supply.
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Thank you for the quick reply. I actually had the same question a couple of days ago and just stumbled across this thread.
It might be a dumb question, but in which cases should I use the normal +3V3 pin rather than the filtered one? What’s the benefit of that?
3V3: This pin will output a regulated 3.3V power rail that can be used to power any components outside the Core. (Also, if you have your own 3.3V regulated power source, you can plug it in here to power the Core).
3V3:* This is a separate low-noise regulated 3.3V power rail designed for analog circuitry that may be susceptible to noise from the digital components. If you’re using any sensitive analog sensors, power them from 3V3* instead of from 3V3.
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Thank you for the detailed explanation.
Are there any use cases where I’d want the high-noise (unfiltered) power rail?
To phrase it a little bit dumber: Why is “only” one +3V3 output filtered? Are there any scenarios where I should avoid using it?
For the case where you are powering the Core from a regulated 3.3V supply, you would not want to power the 3V3* filtered pin, because then the ADC reference would not be filtered. It would be powered directly from your potentially noisy source.
If you needed to power something that wasn’t sensitive to noise, you should power it from the 3V3 unfiltered pin to avoid drawing too much currently through the ferrite bead L2, and causing a voltage drop to your device and the ADC reference. L2 can handle a far amount of current though, last I checked… so for most low power things you can probably still power them from 3V3*. Just think about it before you make the choice between the two and you’ll probably have one reason or another for choosing one over the other. If not, stick with the 3V3 pin.
Thank your for your excellent help here. 
I’m pretty new to hardware hacking and working on my first project at the moment (a Qlocktwo clone powered by the Spark Core). My previous (non spark core) attempt failed hardly as I didn’t know/consider decoupling capacitors at all 
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I am trying to make it properly this time and read a lot about microelectronics, but I’m still a little bit afraid that I overlook something on my PCB 
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You’re very welcome! If you want to start a thread for your project and share details, I’m sure people will offer their help and suggestions as you work your way through the project. Good luck! Looks like a fun project 
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@foobarfoox To expand on what @BDub said a little, I’ll kind of explain what the filtering accomplishes.
Digital devices tend to introduce a lot of noise into the power rails of a system, especially microcontrollers or anything that runs at a high speed (which in the analog world can be anything faster than a few kilohertz). On top of this noise, any sort of digital communication (UART, I2C, SPI or even bit banging) can cause small fluctuations in voltage on the rail.
Analog stuff—including the ADC built-in to the microcontroller—is very sensitive to noise and fluctuation, hence you need a clean and stable supply. This is what the 3V3* supply is for. Essentially by running the voltage regulator’s output through a ferrite bead—which acts to remove stray noise—a 10uF polarized capacitor—which helps stabilize the voltage—and finally a 100nF capacitor to further remove noise. This then goes into the VDDA input which powers all the analog circuitry inside the STM32, including the reference for the ADC.
So, to give you some examples of which output to use for which devices:
If you had a temperature sensor that produced an analog voltage output that you’d read with one of the Ax pins—like a TMP36—you’d want to use the 3V3* rail. If you were using a temperature sensor that provided a digital output—like the DS18B20—you’d power it from the 3V3 pin.
If you were hooking up, say, a high resolution ADC or DAC, you’d most likely use both the 3V3* and 3V3 rails, since most of those types of devices have separate digital (VDD) and analog (VDDA) supply inputs.
The rule of thumb is: If it’s a simple device with an analog output, power it from the 3V3* rail. Check the datasheet, if it has more than one power input or the single power input is referred to as VDDA, use the 3V3* rail.
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@timb I have a couple of shift registers (74HC595) and darlington transistors (ULN2003A) as well as a photocell on my PCB. Looks like I’ll connect the former two to 3V3 and the latter to 3V3*. 
Thank you both!
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