Assistance needed in designing filter for a strain gauge AFE

[nickagian]

I am currently designing an analogue front-end for strain gauges but I am a little confused with something. First of all, to explain the chain, you can have a look at the attached schematic. The instrumentation amplifier that I use is the AD623 from Analog Devices. The LPF in front of the amplifier is for suppressing RF interference and is designed as suggested at page 17 of the device's datasheet. Let us assume that the desired sampling rate from the ADC is around 2kHz. My questions are the following:

(1) Since the expected BW of the output signal from the strain gauge bridge is around 1kHz, I wonder how much should be the frequency of the RFI filter before the amplifier. I guess that the cut-off frequency should not be lower than 1kHz. But if this is the case, how can a RF signal of i.e. 50-60Hz from external power supplies can be filtered out?

(2) I am thinking of using an anti-aliasing filter between the amplifier and the ADC, in order to increase the SNR. Is this approach recommended for my case?

 

[BrownOut]

1) 50 - 60 Hz is NOT RF. Isolation from mains power conductors is what's needed for this source.

2) Anti-aliasing is needed for any DSP application. It's not for increasing SNR, but rather for guaranteeing Nyquest sampling criteria.

 

[nickagian]

1) The system is battery powered, so I guess protection from 50Hz is not needed in my case? Is there any chance that a signal noise at this frequency (and not at RF frequencies which indeed are much higher) can be superimposed on the wires connecting the external strain gauge with the PCB?

2) Thank you for your answer.

 

[BrownOut]

1) Is there any chance that a signal noise at this frequency (and not at RF frequencies which indeed are much higher) can be superimposed on the wires connecting the external strain gauge with the PCB?

Yes. Absolutely. Using the instrument amp will help. Also, use a shielded wire for connection, or else a twisted pair.