Which cap has the lowest noise at 0.22-1uF?

noisy cap

which cap has the lowest noise at 0.22-1uF? I am using a X7R and it picks up a lot of noise when I use it couple ac signal for inverting amplifier. Found that out after a lot of probing.

If I remove the cap and leave it hanging, no noise at all.

What cap should I use? this is for pre-amp for electret microphone.

thanks
ahgu

plastic dielectric

The best ones have plastic dielectrics. Their names begin with "poly-"

which one

Polyester(164 items)
Polyester / Foil(57 items)
Polypropylene(130 items)

which one?

maybe NPO caps are better also?

The x7r has much more noise than the metal film resistor I am using? is this generally true? where can I find info cap noise?

npo good

NPO are good also, but in these high values are big and expensive if you can get them at all. The higher the capacitance for a given physical size the worse they are for noise generation caused by the piezoelectric effect.

The various poly types have individual properties that are not important in your application, such as temperature coefficients. If you use leaded parts, remember that the lead end that has a band around the capacitor body is the outside foil. This should be mounted in the circuit towards the node that will produce the least noise problem from capacitive coupling to the outside world. This is usually the lowest impedance node of the two.

There is a McGraw Hill handbook of components. Capacitor manufacturers should have application notes on low noise applications. Ott wrote a famous book on low noise electronics. These may be available at a local library.

lead end

say I go from 2.2 electret mic to 5V bias - opamp input, where the leaded input positioned?

try both ways

You can try it both ways. Since the microphone element is higher impedance than the op amp input plus bias to 5 V resistor, try that side first. This is a second order effect. If you shield the whole amplifier you will have another layer of hum and noise rejection. (No pun intended.)