What would you say about 25$ single board computer ? Now it has become a reality with Raspberry Pi, a low ARM GNU/Linux box.
It has a quite impressive set of features considering its price. It’s based on BCM2835 SoC which contains a 700 MHz ARM processor and a Videocore 4 GPU capable of HD video
The most popular hobby electronics platforms usually work at 5 V, but many times you need to interface your circuit with a high-end sensor which works at a lower voltage (ex. 3.3 V). It is quite easy to design your own low cost voltage level translators, using only discrete components.
This is an easy once, since it can be done only with passive components. The easiest high to low voltage level translator is a resistor divider.
Since \(R_1\) and \(R_2\) form a resistor divider, the output voltage is only a fraction of the input voltage:
Most of the classical materials teach you to use separate power supplies and connect the digital ground (DGND) with the analog ground (AGND) in a single point. But is it sufficient ? No, it’s not, since you can have high frequency interference that propagates through the ground, from the digital circuits to the analog ones.