The input impedance of the inverting amplifier is essentially
resistor, Ri. The value of this resistance affects the
gain setting of the amplifier as well as the low frequency
rolloff in conjunction with Ci. There is a tradeoff between
having a low frequency rolloff, a high input impedance
and a small capacitor size and value. It is critical to have
a flat band response down to 20Hz while it is equally
important to have a high enough input impedance so that
heavy loading does not occur from the preamp stage.
Using large valued low-cost capacitors implies the use of
leaky electrolytics which affect the output offset voltage.
Electrolytic capacitors are also less linear than other
premium caps and should not be used in the signal path
when not necessary. This tradeoff issue is the toughest
portion of the design. The amplifier gain setting is just as
one would expect for an inverting op amp. Of course, the
input impedance issue can be quickly resolved by using a
voltage follower as an input buffer, ....
