The reference to chopping is really a reference to the use of dynamic element switching; the use of which improves matching between the sink and source current sources. The technique used for this dynamic switching also has the nice characteristic of reducing the magnitude of 1/f noise. The reference to low pass filtering is the optional selection of an LPF within a common mode feedback circuit within the current generator that improves (i.e., increases) common mode input impedance when using a two-electrode solution (i.e., ECG and BioZ amplifiers and drive circuits use the same electrodes). The final option eliminates the common mode feedback, and instead common-mode is established using two resistors.
So, one can look at the four options as follows:
Mode 00: No chopping, thus higher 1/f noise. CFB with low-pass filter, thus high common-mode impedance at 50Hz and 60Hz
Mode 01: Chopping, thus almost no 1/f noise. CFB without low-pass filter, thus lower common-mode impedance at 50Hz and 60Hz
Mode 10: Chopping, thus almost no 1/f noise. CFB with low-pass filter, thus high common-mode impedance at 50Hz and 60Hz. Intuitively, this seems to be the best mode. However, due to the low bandwidth of the CMFB amplifier, stability of the CMFB loop is not guaranteed under all circumstances.
Mode 11: Chopping, thus almost no 1/f noise. No common-mode feedback; the common-mode is realized using two resistors. This mode has high common-mode impedance at 50Hz and 60Hz, but has lower input impedance than the CMFB amplifier, and may exceed the common-mode range for drive currents higher than 32uA due to mismatch.
We believe that the user can optimize for best matching and noise results for their particular application by selecting the option that works best for them. Consider it a toolbox for optimizing the BioZ results.