Electronic – Capacitor of choice for low noise applications


I saw in a schematic a while back that a polypropylene capacitor was used to generate a stable sine wave in an oscillator circuit. I suppose this gave a better 'frequency reference' for the oscillator.

Are some capacitor types better for lower noise applications?

Best Answer

HighK ceramics like X7R, Z5U etc have huge variation of capacitance versus voltage. Using them in filters or any kind of coupling application guarantees humongous distortion. They are piezoelectric: they are both good loudspeakers and microphones. Decoupling a high impedance node with them results in a nice vibration detector. Tolerance on values are not huge, rather they are hyuuuge: expect +20/-50% depending on DC bias. Also, they drift a lot with temperature.

They are truly excellent for power supply decoupling, though, because they are small, have lots of capacitance per volume, low inductance, and are cheap. For decoupling, who cares if it's 1µF +/- 50%?

Now, for filtering applications, or when you run a signal through a cap as in your oscillator application, you want...

  • A known precise value
  • Low temperature drift
  • No capacitance variation with voltage
  • Low sensitivity to vibration

Dielectric absorption and leakage will not matter for your oscillator, but they will for other applications.

Film caps and NP0 ceramic caps are excellent on this, although:

  • large thru-hole film caps tend to be microphonic
  • polyester has worse dC/dV than the other films and NP0

Your first choice should be NP0 ceramics if they are available in the value you need. They are small and cheap, and almost perfect.

NP0 ceramics and High-K ceramics like X7R/Z5U are completely different materials. High-K capacitance varies with DC bias, NP0 does not.