If you do this as an analog method then
it can require some filtering and big circuits.

There are digital methods that work
nicely at low frequencies.

at the feedback summation point its a
DAC summed with the input signal.

For the LPF system, digital averaging
to determine DC is easily done.

A purely analog approach will not be size efficient.

Interesting thing - if the signals are well defined
then your ADC to detect DC offset can be as simple as a
comparator and some DSP. For a DC balanced signal you
are just trying to maintain 50% duty cycle out of the comparator
to get rid of the offset.

A digital counter-averaging system  is orders of magnitude
smaller than an integration capacitor.

Quote:

Considering first order LR with tau at L/R and the desired tau 20sec, even with a resistor 10 Ohms, L should be 200H. If a gyrator has inductance L=C/gm^2, and let's suppose minimum gm value is 5uA/V, C should be 5nF...heh