Miller Technology Inc.  TN095:  Everybody Knows About Capacitor Noise

 

Everybody knows that capacitors, being reactive elements, do not produce electrical noise.   But we do need to take capacitor “noise” and capacitor non-idealities into consideration in some of our designs.

 

It is true that the ESR (Equivalent Series Resistance) of a capacitor will produce thermal, also known as

Johnson noise = .     Fortunately, this is almost always negligible.  The dominant effect of the ESR is usually a small loss factor (finite circuit Q).

 

The remainder of this technical note looks at a four capacitor “noise” sources:  kT/C noise, audible noise, piezoelectric noise, and distortion generation.

 

 

 

 Capacitor “Noise”:

 

The simple circuit fragment shown below implements a lowpass filter on the thermal noise generated by the resistor:

Text Box: Vnoise_out

As is obvious, the lowpass filter cutoff frequency is controlled by the resistor and capacitor values.  From inspection, we note that although the output voltage noise will increase as the value of the resistor is made larger, the lowpass cutoff frequency will decrease.   [3] derives the total output noise for this circuit fragment, and shows the somewhat surprising result:

 

 

In short, the output noise voltage is dependent only on the capacitor value and the temperature.   Thus, in subcircuits that reduce to the equivalent circuit fragment above, the noise floor of the circuit is dictated by the value of the capacitor (and temperature).

 

 


Audible noise from capacitors:

 

Ceramic capacitors are constructed using materials that often are piezoelectric, and the general trend is that the higher the dielectric constant of the material, the more piezoelectric the material becomes.   The typical dielectric constant [2] versus capacitor type is shown below:

Since the materials can be piezoelectric, an applied voltage results in a mechanical strain in the material, so that capacitor types like Z5U and Y5V can produce audible noise.  

 

Side Note:   Inadvertently, this audible noise can occur in switching supply circuits, when the controller is able to slip into “discontinuous” mode.

 

 


Piezoelectric noise from capacitor vibration:

 

The reverse effect (mechanical strain producing a charge output from the capacitor) also occurs.   Values are generally unavailable in the manufacturer’s data sheets, and only a very limited number of journal articles, for example [1].  Briefly, if you wish to minimize this effect, only use NPO or film type capacitors. 

 

 Remember, this concern is not only for audio applications.   For example, if you happen to be designing a PLL (Phase Locked Loop), you certainly do not want equipment vibrations to induce noise into your PLL loop filter, which will result in additional phase noise in the PLL output.

 

The bottom line on the above, is that since capacitor vibration performance is unspecified, you must test to make sure that the components you design in to your application will work as anticipated.

 

Vibration testing of circuits and equipment usually requires big shakers and the inconvenience of hauling your test equipment to where the shaker is located.  In many cases, a small electrodynamic shaker [4] (and suitable amplifier) brings the desired stimulus capability for quick evaluation into your lab and onto your bench.  As an example, the photo below shows the evaluation setup for qualifying some SMT capacitors for a low noise application.

Line Callout 3: Small shaker
(LDS V203 shown – about 3” diameter and 4” long)
Line Callout 3: Circuit and device under test.Line Callout 3: Accelerometer to monitor the applied vibration.

 


Added “capacitor noise” due to harmonic distortion

 

Although not a true noise source, some may indiscriminately lump harmonic distortion products as an unwanted circuit noise.   For sure, it reduces the SINAD (Signal to Noise and Distortion).    The choice of capacitor dielectric type is one important element of low THD (Total Harmonic Distortion) designs.  Most dielectrics will show some amount of change due to applied voltage, which can lead to the creation of harmonic distortion products. 

 

The desire for large capacitance in a small surface mount package size forces manufactures to use less than ideal high dielectric constant materials.   These materials can result in a profound and surprisingly large variance of capacitance with voltage.

For example:  0805 size  X7R vs Y5V 10nF capacitor applied voltage stability (from Murata capacitor data):

For very low distortion applications (THD < 0.002%), use only NPO or film capacitors (such as Polycarbonate or Polypropylene).  

 

 

 

 

 

 


Capacitor Quick Notes:

 

1.  Tantalum electrolytic types have essentially no wear out mechanism.  Long term reliability in a properly designed circuit is excellent.  They are, however, easy to kill with over-voltage, or by failing to restrict the surge current.  AVX suggests a series impedance of at least 0.1 ohm per applied volt.

2.  Aluminum electrolytic capacitors can significantly lose their capacitance at low temperatures.  For instance, at -40C, the capacitance value may be reduced in half.

3.  Restrict the usage of capacitors constructed from PCB material, especially the FR4 dielectric.  FR4 is a generally unreliable capacitor construction material due to its dielectric and thickness variance, drift, the hydroscopic nature of the material, and poor dielectric absorption performance.

4.  In general, the more capacitance you attempt to squeeze into a small space, the more that you will increase the appearance of undesirable side effects like piezo sensitivity, temperature and voltage instability.

5.  Use manufacturers’ supplied programs for viewing temperature and DC working voltage value stability curves (the example below is from Murata capacitor data):

 

0805 X7R vs Y5V 10nF capacitor temperature stability:

 

Capacitor “Noise” References:

 

[1] Nelson &Davidson,  “Electrical noise generated from the microphonic effect in capacitors”.  IEEE International Symposium on Electromagnetic Compatibility.  EMC 2002. Volume 2, Issue , 19-23 Aug. 2002. pp:855 - 860

[2] Sandler & Hymowitz,   “Capacitor Values: Don't Believe the Label”

Power Electronics Technology, May 1, 2007.
[3] Motchenbacher & Connelly, Low-Noise Electronic System Design.  New York:  John Wiley & Sons, 1993.

[4] Ling Dynamic Systems at www.lds-group.com and Bruel & Kjaer at www.bksv.com.