Hi, when I start to bulit my circuit to process the weak signal from magnetic sensor, I wonder if software filter could replace hardware filters. I run notch filter on Labview to attenuate 50Hz noise but output signal amplitude drops slightly. If I should place filter before amplifier. If I must add a hardware filter to my circuit ?

 

Please post a schematic of the circuit you have now for us to evaluate.

ak

 

Generally speaking, a software or digital filter, will require the input bandwidth to be limited to some maximum frequency to avoid a problem know as aliasing. This CAN only be done with an analog filter at the beginning of the chain. There really is no way around it.

 

If your signal is a low-frequency signal (<15Hz) then many of the delta-sigma A/D converters will sample at a rate that cancels out the 50Hz or 60Hz hum. Microchip's MCP3422, for example.
If it's not - then you can design your input amplifier with limited bandwidth (design it as an MFB filter with gain), which will perform the anti-aliasing function.
It is very difficult to place a digital filter before the amplifier, as that would require an A/D that would operate at very small signal levels.
The digital filter is easy - too get rid of 50Hz, just delay the signal by 10ms and add the delayed signal to the sampled signal. That produces a comb filter that will get rid of all the harmonics as well as the 50Hz!

 

Is the 50Hz picked up magnetically from a nearby transformer or is it picked up by an unshielded cable?

 

Or is it really 100Hz from power supply ripple?
Tell us the frequency range of your signal, and we'll be able to help.