low pass filter noise ? best is passive or base on a op amp ?

[Overspeed]

Hello

Some op amplifier provide very low noise but when a low pass filter is added that generate noise ( after amplification OK ) SO :
Does there is designs requirements to built a low noise filter as a low pass 150 Hz filter ?

A common way is to fit a passive resistor + cap at the op amp output as described in Analog / TI docs

Regards
OS

[Benta]

Noise has much more to do with which opamp you select than anything else.
The output RC filter is there for a completely different reason: to make the LPF behave.
Active LP filters turn into HP filters at a certain frequency. The RC output filter cures that to some extent. Sallen-Key filters are especially bad in this respect.
Check the TI application note sloa049d.pdf for a full explanation (p.14).

[Kleinstein]

With good op-amps, just the natural resistor noise can get significant. For lowest noise one may have to use a rel. small resistor and larger capacitor.

[TimFox]

To understand this question quantitatively, one needs to include the source impedance driving the filter circuit.

[Overspeed]

To understand this question quantitatively, one needs to include the source impedance driving the filter circuit.

Hello
That for resistivity material measurement a voltage from a 4 wires resistance setup so resistance is low to very low and I think can be considered as pure resistive
The Op Amp is AD8421 or AD8458 I have also a dual OPA 189 circuit

Regards
OS