Electronic – Why use two 0.1 µF capacitors in parallel

decoupling-capacitor

Below is the schematic of the Pololu A4988 driver board:

There are three capacitors on the motor power input. I can understand why engineers decided to use separate 4.7 µF with larger size (1206, 0.1 µF are either 0805 or 0402): because physically small capacitors have lower ESR, and they effectively filter out higher frequencies (correct me if I'm wrong). But what is the reason to use two 0.1 µF capacitors in parallel? Can they be replaced by a single 0.22 µF in order to save PCB space and cost?

Image of the board; it can be clearly seen that all capacitors are ceramic:

Best Answer

C2 is used to decouple pin 28 and C3 is used to decouple pin 22 (or vice versa): -

See also the Allegro data sheet for the A4988 driver; it shows the capacitors as C7 and C9: -

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Electronic – Decoupling capacitors

I believe the general rule is 1 cap per Vdd pin.

Are you using a ground plane? If so, don't bother attaching the caps to Vss pins. However, if you are using a ground bus instead, then yeah you should attach the cathode of the caps directly to Vss.

Electronic – Adequate decoupling / noise reduction for two accelerometers

The datasheet describes device as very well noise immune one with ratiometric (to power supply) output from DC to 400Hz. So to read DC "ratio" well, try use 2+2+2=6 wire "drive" "sense" cable for 2 devices. At DC you are affected by ohmic, galvanic, thermoelectic forces, but not the radio.

-- Drive --->--*-----+----------------------------+
-- Sense ---<--/     |                            |
                     |                            |
-- Sense ---<----Device (of DC half bridge type),Device--->---Sense--
                     |                            |
-- Sense ---<--\     |                            |
-- Ground --<--*-----+----------------------------+

All 3 sense wires should meet ratiometric input of ADC, say top goes to positive reference input of ADC, middle into positive input, bottom sense goes into 2 pins: negative reference (of balanced input) and negative pin of input.

Edit: Other thought. If there is movement on flexible cable, think about "triboelectricity". If cable friction, hit, vibration is an issue, then read specs about correct impedance of load. If this is high impedance, then add differential "line drivers" with current loop output and match a type of ADC to be "current ADC". Or better add small ADCs on board itself.

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Electronic – Decoupling capacitors

Electronic – Adequate decoupling / noise reduction for two accelerometers

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