What is the point of these two parallel capacitors in this schematic?

Sensacell

Joined Jun 19, 2012
3,327
This is often done to improve the effectiveness of capacitor "bypass"
Different types of capacitors perform better at different frequencies, they choose caps that have overlapping characteristics to flatten out the response over a larger band of frequencies.
 

Thread Starter

akke

Joined Dec 17, 2015
77
Ok. Thanks for clarifying that ;-)

Other thing:
I connected this breakout board to my arduino MKR (3.3V) while supplying this board with 6V for the motor. I didn't think about the fact that my arduino was 3.3V and the board might be communicating with a higher voltage over the SDA/SCL lines.

While everything works perfectly fine, I do see that they pullup the SDA/SCL lines to 6V with a 10K resistor and I do think the DRV8830 chip is working at VCC level for SDA/SCC too. So I probably am lucky I didn't break the arduino?
 

crutschow

Joined Mar 14, 2008
33,325
At the bottom of the schematic there's a 22uF (ceramic? as the schematic has no polarized symbol?)
It is likely a polarized electrolytic as ceramic caps aren't built that big.
It could be a physically large film capacitor, but that is not common in such applications.
 

MrChips

Joined Oct 2, 2009
29,802
Why do circuit designers put 0.1μF in parallel with 10μF capacitor? That makes the effective capacitance 10.1μF.
But the 10μF capacitor likely has a tolerance of 10%, i.e. the actual value could be anything between 9μF and 11μF.
Hence the additional 0.1uF makes no difference in the overall capacitance value.

The answer lies in the frequency response of each capacitor. The 0.1μF capacitor is better at attenuating high frequency noise.
 

dl324

Joined Mar 30, 2015
16,111
At the bottom of the schematic there's a 22uF (ceramic? as the schematic has no polarized symbol?) capacitor in parallel with a 0.1uF ceramic capacitor.
The 22uF is an electrolytic capacitor. Its parasitic inductance will limit its effectiveness at higher frequencies. The 0.1uF ceramic capacitor will have little effect at lower frequencies, but it's lower parasitics will make it more effective than the electrolytic capacitor at higher frequencies.

At even higher frequencies, you'd benefit from paralleling 0.01uF and 0.1uF ceramic capacitors.
 

dl324

Joined Mar 30, 2015
16,111
Mr Bogatin says, the parallel caps are an old, outdated reflex. Today's ceramics are large enough
Physically larger ceramic capacitors are still going to have more parasitics than smaller (physically) capacitors. And a 47uF ceramic cap is going to be huge.
 

KL7AJ72

Joined Apr 15, 2021
22
Hello,

I'm looking at a the schematic of a DRV8830 breakout board from pimoroni here ( https://cdn.shopify.com/s/files/1/0174/1800/files/DRV8830_breakout.pdf?v=1646733167)

At the bottom of the schematic there's a 22uF (ceramic? as the schematic has no polarized symbol?) capacitor in parallel with a 0.1uF ceramic capacitor.

I really wonder what the point is of having them in parallel?
If you should happen to get your hands on a NanoVNA (and they're so cheap now, there's no excuse for any electronic nerd not to have one), check the response of a typical capacitor between a few hertz and a few hundred megahertz' You wouldn't believe all the weird resonances that show up. It's amazing any commercial capacitor works at all!
 

Veracohr

Joined Jan 3, 2011
771
Check out the link posted by @crutschow. There s a 47uF ceramic in 0805 size. Multilayer ceramic capacitors MLCCs are quite small.
Small package sizes that are large in capacitance generally have higher drop in capacitance with DC bias than the same value in a larger package. There’s always tradeoffs.
 

KL7AJ72

Joined Apr 15, 2021
22
By the way, at UHF, it's common practice to use parasitic inductance for SERIES resonance in bypass applications. There are charts that show the optimum lead length of bypass capacitors at UHF.
 
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