lets say my temperature sensor outputs about 100mv analog output and is combined with noise.
What's your wanted f-3dB
and how much filtering do you need?
With a temperature sensor, the frequency will be low (and the larger the mass of the sensor, the lower the cut off frequency can be).
[...] configured as an amplifier with about gain of 25.
That would only bring it to 2.5V and as such loose a single bit of resolution.
1.) how what is the most appropriate way to cascade the filter and the amplifier
sensor ----> low pass filter ----> amplifier
sensor ----> amplifier ----> low pass filter
That depends...but placing the filter after the amplifier will reduce the noise of the amp as well.
or is there any method that combines thee filter and amp stage together so that single op amp can be used.
How steep a filter do you need?
2.) what type of low pass filter should i use?
salen-key or RC filter with buffer amplifier
The Sallen-Key topology is
an RC filter, but perhaps you are thinking of a passive RC filter followed by a buffer op-amp?
In the Sallen Key, you filter on both the direct signal and the feedback, so you can make a better (steeper) filter that way.
As your pass band will likely be from (close to) DC to less than 10Hz (assuming either an NTC, a diode or similar sized device) and the noise will likely be magnitudes higher, perhaps a simple 6dB/octave filter may be quite enough, but if your signal and the noise is fairly close, a Sallen-Key with Chebyshev type 1 polynomials might be the best option (short of a fast DSP, where you can make brick wall filters).
You could split the work between hardware and software, with a soft filter in hardware, just to kower the noise a bit and then use repeated sampling, to remove as much of the noise as your timing allow. A signal that doesn't change (within the time frame for the multiple samples will average to the correct value, while random noise will cancel to a degree determined by the number of samples averaged.