difference between analog power and digital power

Question

Could 3.3v pin of the STM32F4-discovery board be an analog power source? If not could I use a regulator for converting it as an analog power source?

I'm making a camera module using an image sensor. Its datasheet says that you need analog 3.3v, digital 1.8v and digital 1.2v source for power supply. What I wanted to do is to connect this camera module to STM32F4-discovery. Since the boards has 3.3v output pins and GND pins, if I use regulators for converting 3.3v to 1.2v and 3.3v to 1.8v, it seems I can provide 3 different voltages to the sensor. However, after searching for a while I realised that analog/digital powers are not same. But I couldn't understand so far exactly what is the difference and whether the 3.3v output pin on the discovery board could be an analog power source.

Answer 1

There is no real difference between an analogue power supply and a digital power supply. They are separated because the digital power supply will supply current to digital switches which will switch on and off rapidly, putting large current demand spikes onto the power line, and hence cause momentary voltage drops and spikes on the line - these spikes are managed by decoupling capacitors local to the switching devices, and bulk capacitance at the power supply.

By keeping the two supply lines separate the circuit designer can isolate these current spikes from analogue circuitry which may be affected by small voltage drops - for example if the supply is used as a reference for an A2D. Often the two supplies are fed from the same source, but the analogue supply may have a small RC or inductive filter in line to remove digital noise present at the source.

So yes, the 3.3v pin of the STM32F4-discovery board could be used as an analog power source. I would just add a small series resistor (less than 0.05V drop at the current you are using) and additional capacitance to ground - to give a first order low pass filter frequency of about 1KHz - or whatever you deem appropriate.

Answer 2

As an addition to Icys (correct) answer:

The made a point on Voltage Drops on Analog Circuits: If you have a analog power level powering your complete analog elements, you often take small voltage drops out of the calculation. If the analog Voltage drops by 5%, this means that the Signal Voltage (eg. Output of a simple temperature sensor like a NTC) will drop by 5%. If you have a common analog power including the ADC Supply (and therefore your reference voltage), the ref voltage will also drop by 5%, resulting in a constant value on the output of the adc.

This should be kept in mind when designing circuits which may be under effect of voltage drops, eg direct battery powered devices. If your ref is the internal 1,8v ref, it may not drop and alter your measured values over the discharge of the battery as your signal from ntc may drop.