I am designing the filtering for my measurement signal chain, are there rules of thumb for which filter order that I should use?
The order of the antialiasing lowpass filter, LPF, depends on the ADC sampling frequency, fs, the input signal bandwidth, SignalBW and the magnitude of out-of-band signals or noise producing aliasing errors. The highest aliasing errors are out-of-band signals or noise in a frequency zone around fs and equal to 2xSignalBW (a worst-case aliasing zone is fs ± SignalBW). Any signal in the aliasing zone will fold back into a lower frequency and will be undistinguishable from a legitimate input signal.
The purpose of the antialiasing filter is to reduce the amplitude of the signals in the aliasing zone so that, when they fold back in the SignalBW, they are kept well below the legitimate input signal. The higher the LPF order the higher the stopband attenuation in the aliasing zone. In an oversampled signal chain, fs is much higher than SignalBW (fs at least 10xSignalBW) a 2nd or 3rd order LPF may provide adequate attenuation at ≥fs (-40dB and -60dB for a 2nd and 3rd order Butterworth LPF respectively when the -3dB corner is equal to SignalBW). In case fs is very close to SignalBW, for example fs=5xSignalBW, then a 4th or 5th order LPF maybe required to provide at least -50dB to -70dB attenuation at fs. Also, placing the -3dB frequency of the antialias filter at the SignalBW may introduce unacceptable magnitude droop and phase distortion in the SignalBW. In that case the -3dB frequency needs to be increased, which in turn will require a higher order antialias filter.
For oversampled sigma-delta converters, such as the AD7768, the actual fs is usually much higher than the input signal bandwidth of interest. This usually makes it possible to use lower order filtering in front of the converter, or filtering with a flatter response beyond the signal bandwidth of interest, which is one of the benefits of the medium bandwidth sigma delta converters. The datasheet for the converter usually explains the aliasing profiles in detail.
Finally, oversampled continuous time sigma-delta, such as the AD4134, feature an embedded anti alias filter and in many cases do not require any additional filtering.
For a more detailed discussion on technologies to address the aliasing requirements of precision signal chains see the following article: AC and DC Data Acquisition Signal Chains Made Easy | Analog Devices
For a more detailed discussion on the design challenges and considerations associated with implementing analog and digital filters in ADC signal chains, to achieve optimum performance, see the following article: Practical Filter Design for Precision ADCs | Analog Devices