Electronic – Difference: Low Pass Filter and Large Capacitor

adcfilterlow pass

When working with microcontrollers it is recommended to place filter/decoupling capacitors between a supply pin and ground. I understand the purpose of this implementation, namely that voltage across a capacitor cannot instantaneously change, but what are the outstanding differences between a singular capacitor and a low-pass filter?

These are not calculated values; I inserted this just as an illustration.


simulate this circuit – Schematic created using CircuitLab

For example, if I were wanting to provide my ADC reference voltage a clean supply to compare input voltages against, I could either realize a low-pass filter to reject the high-frequency fluctuations or simply insert a adequately sized capacitor.

My immediate thought is that the initial current demand of a singular capacitor could momentarily exceed the maximum rating of the MCU, but with a resistor this current would be limited. Would it not be that with a LPF (with a resistor) that one could potentially design such that the output impedance of the filter is infinite as to not load the ADC? Similarly, a capacitor alone would provide sufficient voltage filtering but would it not result in a low output impedance?

What are the pros and cons of each realization of filtering, and when should a designer use one or the other?

Any other thoughts ?

Best Answer

The cap near the power pin is not to protect the part from noise, but to keep the part from generating noise as the logic switching causes rapid changes in supply current. Ideally the cap would supply instantaneous demands for more current without increasing current all the way back to the power source.

The sum of the impedances on the PSU side of the circuit - the internal impedance of the PSU plus the inductance, resistance, and capacitance of the traces or planes - is enough to give some low pass filtering on the input side of the cap. I think of the cap as a tiny a power supply that can respond to demands with a bandwidth in the multi-MHz range. The larger regulators that supply a full circuit react far too slowly and the cap is a temporary source of power that replaces or bypasses (or decouples) the PSU. Placing the cap close to the power pin on a chip minimizes resistance and inductance that would slow the response.

CMOS parts consume most of their power while switching state. For microprocessors this means on clock edges and the current draw is in little fast spikes. The size of the spikes varies as fast as the clock as every instruction uses different combinations of internal circuits. Imagine the circuitry used in checking a register for zero versus fetching data from RAM. The power needed fluctuates at the clock rate. The greater the current changes, the bigger the cap. Calculating the right size is a matter of estimating for most of us and the 0.1uF ceramic cap is so common that it is very low cost. Capacitor construction is also a concern as well as change with temperature. Some can respond quicker than others and some vary by 80% over the commercial temperature range.

They are also called bypass caps because: 1) They can "bypass" (short) high frequency PSU noise to ground. 2) They can "bypass" the PSU and respond to high frequency demands for power.

Also called "decoupling caps", a more accurate term for high frequencies as they "decouple" the power demand between the part and the PSU.