Do slow op amps filter out noise?

[AQUAMAN]

If I have an op amp with a BW of 5khz into an ADC, will that op amp filter out noise at say 1Mhz?

[penfold]

The short answer is generally yes. But I wouldn't rely on that as an excuse not to filter out noise as early as you can in the signal chain. There are still ways in which HF noise can be troublesome and ways that parasitic elements of the resistors can cause the gain to increase slightly at HF... but whether that is a problem to your circuit is rather circumstantial.

[Vovk_Z]

Yes and no. It depends. :-)

[T3sl4co1l]

Opamps can rectify high frequencies.  Classic example is old bipolar amps like LM358, in the presence of frequencies over 100MHz (probably over 10MHz even).  The input stage acts like a diode into a capacitor, rectifying RF.  Thus, RF adds to the signal like a voltage offset.

JFET and moreso MOSFET types are less prone, and many precision amps these days have some manner of internal filtering, which the datasheet reports (immunity at fractional GHz and up).

The best idea is always to filter your signal down to only what's needed.

Tim

[David Hess]

Limiting operational amplifier bandwidth limits noise, and some amplifiers, like the LT1097, give access to an "overcompensation" pin which allows this.