Capacitor of choice for low noise applications

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.

Film capacitor are widely used in applications that require high stability of the capacitance value. In audio, they're used for the signal path, whereas electrolytic or ceramic are used for bypassing.

Film capacitor lack the parasitic piezoelectric effect present in ceramics, and they also are very stable with respect to its bias voltage. In ceramics it's the other way round: capacitance can change up to -90% with bias voltage, introducing enormous nonlinearities if used in the signal path.

I saw this in the LT6657 datasheet:

For very low noise applications, film capacitors should be considered for their lack of piezoelectric effects. Film capacitors such as polyester, polycarbonate and polypropylene have good temperature stability. Additional care must be taken as polypropylene have an upper limit of 85°C to 105°C. Above these temperatures the working voltage often needs to be derated per manufacturer specifications. Another type of film capacitor is polyphenylene sulfide (PPS). These capacitors work over a wide temperature range, are stable and have large capacitance values beyond 1μF

And from a manufacturer

Polypropylene is generally selected for its excellent dielectric characteristics (losses, absorption, dielectric strength, insulation resistance:

• very low Tgd and dielectric loss,
• low dielectric absorption,
• excellent dielectric strength,
• high insulation resistance,
• temperature- and frequency-stable characteristics,
• excellent self-healing properties for metallized Polypropylene,
• etc.