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Saw a Youtube video from TI talk about opamp noise and one of the points they made was limit your bandwidth. TI:How to select an op amp based on datasheet noise specs

While I do understand the reason for limiting the bandwidth, I've seen a lot of schematics where there is no input filter. And in DC signals, I don't think I've seen any filter.

Is it good practice to put input filters (passive or active) to any opamp circuit if board space and cost can be neglected ? Or is purely driven by requirements and anything more than what is required (even if the signal looks cleaner) is just "over engineering" ?

A dramatic analogy would be taking a helicopter to the grocery store, simply because you can vs doing the minimum required that gets you there in a reasonable amount of time like car, taxi, bus or walk.

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  • \$\begingroup\$ This probably depends on under what conditions you want your circuit to work, if they can be guaranteed and if those will then always work without the filter. \$\endgroup\$
    – PlasmaHH
    Feb 22, 2015 at 21:54
  • \$\begingroup\$ P.P.Eckersley, the BBC's first Chief Engineer in the 1920s, is reputed to have said "The wider open the window, the more ***t flies in". Still true 90-some years later... \$\endgroup\$ Feb 22, 2015 at 23:13

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While I do understand the reason for limiting the bandwidth, I've seen a lot of schematics where there is no input filter.

The inherent noise from an op-amp, as a first approximation (e\$_{NOISE}\$), cannot be reduced by using an input filter. It can be reduced by an output filter or filtering in the feedback loop to reduce the noise bandwidth of the circuit but it cannot be removed by input filters.

Taking this to a more accurate approximation, some inherent op-amp input related noises can be reduced by input filtering such as i\$_{NOISE}\$. These for many circuits will be less than the e\$_{NOISE}\$ and are usually reduced by choosing smaller value resistors around the input stages of a circuit. This also reduces inherent thermal noise from resistors because the resistor values are much smaller.

It's sometimes good practise to limit the frequency response of an op-amp circuit to that needed for performance/spec reasons but there are many instances when this won't be the case such as when the op-amp is inherently limiting the frequency response i.e. you are pushing a design to its limits at high frequencies.

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