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Power supply bypass capacitor types
Power supply bypass capacitor types
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Old 13th November 2013, 12:55 PM   #1
shzmm is offline shzmm  Germany
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Default Power supply bypass capacitor types

Sorry if this is a topic that appears a lot but I have a question about smaller value bypass capacitors and I am finding conflicting information.

My project is a simple microphone preamplifier with an OP627 opamp. In the datasheet provided by Burr Brown, it is suggested that "applications with low impedance loads or capacitive loads with fast transient signals demand large
currents from the power supplies. Larger bypass capacitors such as 1F solid tantalum capacitors may improve dynamic performance in these applications."

All well and good, but I have always used film capacitors, or ceramic in lower values, in this position-- as close as possible to the pins. I did this because they were cheap and I've seen other builders and designers using them so I figured it was acceptable practice. However, upon reading Analog Devices MT101 decoupling techniques tutorial, they do not recommend using film capacitors: "In general, film type capacitors are not useful in power supply decoupling applications because they are generally wound, which increases their inductance. They are more often used in audio applications where a very low capacitance vs. voltage coefficient is required."

So when users here and elsewhere (i.e. Rod Elliot for one) recommend using film bypass caps is this actually a misunderstanding on my part? Does it depend more on the application? And what would a cost effective alternative be to film be in 1uF value? (I've never actually used tantalum as it was more expensive and had read bad reports on the quality.)
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Old 13th November 2013, 03:09 PM   #2
DF96 is offline DF96  England
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Everything in electronics depends on the application. 'Design by rule of thumb' often works, but not always. The first question I would ask is "what do Burr Brown mean by 'fast transient signal'? This could be significantly faster than anything you would see in an audio application.
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Old 13th November 2013, 03:42 PM   #3
fpitas is offline fpitas  United States
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I use small caps as close as practical to the pins, followed by successively larger caps. I might start with a 1000pF ceramic 0402, followed by a 0.1uF ceramic 0603, and if it's a high-current situation as you mention, maybe a 2.2uF ceramic 0805. I also use small isolation resistors to the plus and minus supply rails, to decouple high frequency transients. If the op-amp draws too much current to allow more than a few ohm isolation resistor, I use an appropriate ferrite bead instead. The overall technique has worked on a lot of op-amps, even ones with 5GHz unity gain frequencies.
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Old 13th November 2013, 04:58 PM   #4
gootee is offline gootee  United States
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Film caps are wound but not necessarily very inductive. It depends on the type of construction. The better ones are wound with the two metal layers offset so that they protrude from the sides of the roll, slightly, one from one side of the roll and the other from the other side, with end caps that make contact with the entire length of the long edges of the rolled metal layers. The worse ones would connect the leads only at one end of the roll, to the short edges of the metal layers.
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Old 13th November 2013, 05:09 PM   #5
AndrewT is offline AndrewT  Scotland
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I believe from my researches that ESR is very important to decoupling effectiveness.
Film capacitors in general are very low ESR.
Electrolytics again in general are medium to high ESR.
HiK ceramics are a middle ground that have medium ESR.
These x7r seem to make good decoupling capacitors the highest frequencies.
Electrolytics seem to make good decoupling for medium frequencies.

Film capacitors tend to be very good at passing signals. That "very good" just about bans them from decoupling duty.

As I said at the beginning, this is what I believe, but there are certainly experts on the Forum who may have different views.
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Old 13th November 2013, 05:31 PM   #6
fpitas is offline fpitas  United States
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ESR is important, but the inductance, and the resulting self-resonant frequency, is very important for bypass use. It's very hard to beat surface mount ceramic caps for low inductance.
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Old 13th November 2013, 05:31 PM   #7
Mark Johnson is online now Mark Johnson  United States
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Power supply bypass capacitor types
Here are some plots for EPCOS thru-hole film capacitors, and TDK thru-hole ceramic capacitors. See for yourself how big the difference is.
Attached Images
File Type: png film_caps.png (60.6 KB, 812 views)
File Type: png ceramic_caps.png (378.6 KB, 792 views)
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Old 13th November 2013, 06:06 PM   #8
AndrewT is offline AndrewT  Scotland
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ceramic through hole from those plots show 1uF 0r011 @ 1M5Hz and 0.1uF 0r030 @ 5M2Hz
whereas film shows 1uF 0r018 @ 1M2Hz and 0.1uF 0r034 @ 5M1Hz.

That seems to indicate that these two examples are very similar, rather than very different.

Could the leads of the through hole have such high inherent inductance that this swamps the other parameters?
Would this indicate even more that short leads are important?
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Old 13th November 2013, 06:17 PM   #9
fpitas is offline fpitas  United States
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Short leads are very important. No leads is much better.
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Old 13th November 2013, 06:18 PM   #10
tinitus is offline tinitus  Europe
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I thought it was a matter of dealing with ripple at a certain high frequency
all of which is only possible to determine by measurements

and without proper measurement, its better not to do it at all

actually, I might have read this in a Leach paper/article
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