For a 12-bit accuracy, the input signal frequency f should be less than 0.0156fu. fu is the -3dB cut off frequency of the close loop.
f=0.0156fu . if \[ \tau \] is the time constant, so \[ \tau \] =1/fu.
1/f=1/(0.0156fu), we have 1/f=t1, 1/(0.0156fu)=64\[ \tau \], so t1=64\[ \tau \] .