Minimizing RC area for given Time Constant

20 August 2019 Link


During IC design it is often required to choose appropriate resistor and capacitor sizes to implement a filter or design networks. Here we derive a formula to decide what resistor and capacitor values can be used for a given time constant so that the area is minimized.


$C_□$ = capacitance per unit area for the process
$R_□$ = resistance per square for the process
$τ$ = Time constant of the circuit = $RC$

Here W = width of the resistor, this is chosen based on the accuracy needed for the resistor according to the process data
Differentiating with respect to C we have:
$${d{Area} }/{dC} = -τ/{R_□}W^2 1/{C^2}+1/{C_□}$$

Equating that to 0 and calculating C we have:

Thus using this capacitance value and using $R=τ/C$ would give the optimized area for the RC network