length of all the MOSFET's in the design are kept at 2.5*L_min (roughly 2-3 times of L_min)... and Some other design have follow some other strategy like for "Digital blocks length is fix at L_min and for analog blocks it is 2.5*L_min.
For example:
In 180nm Technology
for Digital blocks length is at 180nm fixed...
for Analog blocks Length is fix at 500nm (which is 2-3 times of Lmin)....
Reason for choosing different lengths in analog and digital designs is that
1. In analog design we would be choosing the length based on matching requirements. Minimum length would have poor matching i.e more reflections, so larger lengths are used to get around 50 Ohm characteristic impedance (Perfectly matched). For some parts of the circuit where matching is not critical
and large gm or high current handling are required we can use minimum length.
2. has higher channel length modulation which will degrade the mos transistor output impedance and hence degrade the design of the current mirror/reference circuits and op-amp.
