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Are your military ceramic capacitors subject to the piezoelectric effect? |
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Description :
Are your military ceramic capacitors subject to the piezoelectric effect?
Answer :
Certain classes of ceramic capacitors exhibit a normal characteristic, called piezoelectricity, than can cause unexpected effects in certain circuits. In some cases, the piezoelectric effect may result in the appearance of electrical noise, while in other cases, an acoustic sound may be heard, coming from the capacitor itself. Ceramic piezo effects are well known, and were even the basis for the ceramic phono cartridges used in the past.
Piezoelectricity is a common characteristic of many ceramic chip capacitors and occurs in those classes of dielectric which are classified as ferroelectric. Piezoelectric effects can result in noise for ferroelectric ceramic chips, such as those used for military BX & BR, as well as commercial EIA Class 2 and Class 3 dielectric, such as X7R, X5R, X8R, Y5V, Y5U, Z5U, etc. Piezoelectricity occurs in all ferroelectric dielectrics, regardless of manufacturer. Note that there are essentially no piezoelectric effects in Class 1 capacitors, such as C0G, NP0, or military BP - none of which are ferroelectric.
Piezoelectric noise is only occasionally an issue, since it is low level. However, it can show up in specialized applications subject to mechanical stress of the ceramic during shock, vibration, compression, and torsion. Examples include high gain pre-amps, hand-held microphones at rock concerts, and monitoring equipment subjected to sudden shock or heavy vibration. When it occurs, most piezoelectric noise is in the 3 KHz to 30 KHz ranges, although detailed studies have not been done over a wider range.
The piezoelectric effect is tied to the crystal structure of the dielectric. In ferroelectric materials, the crystal structure tends toward the tetragonal, with the Ti cation located at a non-centered position in the crystal. This results in an electric dipole structure. When this structure is mechanically deformed, the charge center of the crystal shifts, producing a dipole moment and polarization. This results in the appearance of a voltage at the capacitor terminals. That voltage increases as the mechanical deformation increases. This can be a design issue in high gain amplifier circuits subject to mechanical vibration or sudden impact, since these piezoelectric voltages could be coupled into the circuit, introducing errors.
The complementary effect also occurs, in that electrical stimulation of ferroelectric compounds can result in mechanical deformation. In circuits which operate at acoustic frequencies, the capacitors will tend to respond and may emit acoustic noise. As the frequency goes up, the capacitor can no longer respond, and the acoustic noise will be damped out
Remedies depend upon the operating constraints of the designs. Use of a different capacitor type is one obvious approach, and may be the only solution for low frequencies. Other possibilities include (must be evaluated by the customer, based on circuit requirements):
- Use a different dielectric - C0G can replace X7R for low cap values, if the package size increase is acceptable.
- Use a different type of capacitor, such as tantalum (the 1206 0.1 uF 50 volt ceramic can sometimes be directly replaced by a tantalum equivalent, the 3216.
- Use a leaded part, rather than SMT - the leads tend to decouple the mechanical stress from the chip
- Use a smaller footprint SMT part, to minimize the span on the board, which can help to isolate the chip from flex and vibration effects
- Minimize vibration on the board by changing the board mount system, adding dampening materials near the chip, or by relocating the chip.
- Use a part with thicker dielectric, usually corresponding to a higher voltage rating. This reduces the voltage gradient, which reduces piezoelectric noise, if the package size increase is acceptable.
- Use a chip with greater overall thickness, which helps to prevent physical distortion and stress.
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