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Old 21st March 2006, 05:02 AM   #1
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Default Power Supply Decoupling

What is the best three capacitor combination for power supply decoupling on amp PCBs? I know to use three caps (one large, one medium, one small), but I get conflicting results as to what kind the medium and small ones should be. This is what I can gather so far:

Large: electrolytic (10uF)
Medium: film or poly (but what kind of film or poly? 1uF)
Small: ceramic or mica (100pF)

What have you all found that works best (and is easy on the wallet)?
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Old 21st March 2006, 06:49 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
my view is use the biggest polyfilm you can afford and keep this and the ceramic right at the last connection point before leaving the board.

Remember to add mains caps (X & Y rated) and switch suppression.

You might also want to experiment with a snubber on the output and on board the PCB.
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Old 21st March 2006, 07:18 AM   #3
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What are you decoupling? If it's op-amp circuits I would just use 100nF ceramic or film. When you start paralleling different types and sizes of caps there is a possibility that their ESR and ESL react together and cause undesirable effects.
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Old 21st March 2006, 07:26 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi Richie,
That's a bypass you have described, designed to absorb the bulk of the supply line glitch when the opamp changes current consumption.
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Old 21st March 2006, 03:06 PM   #5
Eva is offline Eva  Spain
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Paralelling dissimilar capacitors directly does not produce a lower impedance PSU, since each capacitor resonates with the rest producing peaks in the impedance verses frequency characteristic of the PSU that may reach 10 ohms or even 100 ohms and may be an octave wide.

A single medium sized (modern) electrolytic capacitor (around 100uF) will do a better job and produce a nice flat and resistive low PSU impedance up to at least 1Mhz. On the other hand, PCB track inductance of the typical dumb amplifier layout (rails in the sides and ground in the center of a wide PCB) is far more inductive than any electrolytic capacitor, so the only way to reduce PSU impedance above 1Mhz is to learn to lay out PCBs properly (most audio designs are terrible).
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Old 21st March 2006, 03:21 PM   #6
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Quote:
Originally posted by Eva
Paralelling dissimilar capacitors directly does not produce a lower impedance PSU, since each capacitor resonates with the rest producing peaks in the impedance verses frequency characteristic of the PSU that may reach 10 ohms or even 100 ohms and may be an octave wide.

A single medium sized (modern) electrolytic capacitor (around 100uF) will do a better job and produce a nice flat and resistive low PSU impedance up to at least 1Mhz. On the other hand, PCB track inductance of the typical dumb amplifier layout (rails in the sides and ground in the center of a wide PCB) is far more inductive than any electrolytic capacitor, so the only way to reduce PSU impedance above 1Mhz is to learn to lay out PCBs properly (most audio designs are terrible).
FWIW, Analog Devices recommends 100nF ceramic in parallel with 1 tantalum for low-noise measurement applications.

The real world problem with all the parasitics is illustrated below:
Click the image to open in full size.

here's one layout which they recommend.
Click the image to open in full size.

The entire article is referenced here:
http://www.analog.com/library/analog...09/layout.html
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Old 21st March 2006, 03:34 PM   #7
Eva is offline Eva  Spain
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This article is useles for our purpose, as it ignores completely mutual capacitor interaction and does not even model capacitor inductance in the drawings!!

Click the image to open in full size.

First, this graph shows that a 2.2uF tantalum has very good high frequency performance, as it does not show much higher impedance than the ceramics at higher frequencies (it may be an artifact of the measurement setup, though).

Also, a resonance peak will be formed when paralelling the 2.2uF and the 100nF, it will be located where both impedance graphs cross, at 10Mhz, and will probably produce a peak of 1 ohm or more, thus ruining the small advantage that the 100nF could have provided.

In other words, the PSU impedance will be actually *lower* when the 100nF paralell capacitor is not used. Note that the 2.2uF tantalum provides by far the lowest impedance in the 1Mhz to 10Mhz range, that is critical for audio amplifier stability since it's where most circuits transition from feedback to no-feedback, while the rest of the capacitors show much higher impedance here.
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Old 21st March 2006, 03:55 PM   #8
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Texas Instruments (Burr Brown) also recommends two bypass caps in their DSP application notes.
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Old 21st March 2006, 04:37 PM   #9
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Eva
This article is useles for our purpose, as it ignores completely mutual capacitor interaction and does not even model capacitor inductance in the drawings!!

Click the image to open in full size.

First, this graph shows that a 2.2uF tantalum has very good high frequency performance, as it does not show much higher impedance than the ceramics at higher frequencies (it may be an artifact of the measurement setup, though).

Also, a resonance peak will be formed when paralelling the 2.2uF and the 100nF, it will be located where both impedance graphs cross, at 10Mhz, and will probably produce a peak of 1 ohm or more, thus ruining the small advantage that the 100nF could have provided.

In other words, the PSU impedance will be actually *lower* when the 100nF paralell capacitor is not used. Note that the 2.2uF tantalum provides by far the lowest impedance in the 1Mhz to 10Mhz range, that is critical for audio amplifier stability since it's where most circuits transition from feedback to no-feedback, while the rest of the capacitors show much higher impedance here.
What makes you think that 2.2uF graph is for tantalum? Looks like MLCC surface mount ceramic cap to me.

2.2uF tantalum with 10mOhms ESR? No Have, Sir.

What comes to paralleing lets say 10uF tantalum+ 100nF ceramic, there is usually NO resonance as tantals have enough high ESR to damp any problems like that.

Paralleing 100nF ceramic with 10uF foil or ceramic would be a bad idea just like Eva said.
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Old 21st March 2006, 05:04 PM   #10
Eva is offline Eva  Spain
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Indeed you will find a lot of people recommending paralell capacitors, but they will do it without analysing capacitor interaction (most of them without ever knowing these effects).

I have just taken the following capture from one of my circuits. It's low power logic and it's made with a 7555 CMOS timer and some 4000 CMOS gates, all them together can only draw approx. 50mA transients at best from the PSU (so that's nothing in comparison with what a DSP can do to a resonant PSU).

As supply decoupling I have used a 22uF 35V standard electrolytic (high ESR so less likely to ring) and a 100nF multilayer ceramic paralelled quite closely. The measurement is done directly across the legs of the 100nF ceramic with a common mode filter inserted in the oscilloscope probe to remove common mode noise.

See how happily the two paralelled capacitors are singing their 2Mhz song after each logic transient

Click the image to open in full size.

BTW: The ringing disappears if either capacitor is removed.

All that discussion is just a matter of lazyness, it's very easy to recommed stacking dissimilar capacitors just because you have read about it in some paper or forum, but whithout having ever investigated the resulting effects on supply impedance.
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