Precise capacitor in parallel to power supply

Question

I found this schematic in ADNS-2610 datasheet.

There are two capacitors in parallel between pins 6 and 7 (GND and V_DD.) One is 4.7 uF and the another is 0.1 uF, so the parallel result is 4.8 uF.

Why is the 0.1 uF capacitor needed? It doesn't seem to be a precision-critical component, because it's just in parallel to the power supply (I think just to stabilize the voltage). Isn't just the 4.7 uF one sufficient?

Maybe the fact that one is polarized and the other isn't makes some constructive/technical difference? I can't see any reasonable motivation.

Answer 1

If capacitors were ideal, then there wouldn't be much point in it.

Real capacitors have resistance and inductance, however.

In this case, it is the equivalent inductance that is the problem.

That 4.7µF capacitor will be a fairly large electrolytic capacitor that is made up of rolled foil. Its inductance will be fairly high, and prevent the capacitor from responding to high frequencies.

The smaller capacitor will be a ceramic capacitor with low internal resistance and very low inductance. It can respond to very fast changes in current that the electrolytic capacitor can't catch.

• Large electrolytic capacitor to handle long(ish) incidents.
• Small ceramic capacitor to catch fast transients.

The both together will do a better job of keeping the supply voltage clean than either alone would do.