This circuit uses an LM11
to form a voltage follower with 1G ohm input resistance
built using standard resistor values. With the input
disconnected, the input offset voltage is multiplied by
the same factor as R2; but the added error is small
because the offset voltage of the LM11 is so low. When
the input is connected to a source less than 1G ohm, this
error is reduced. For an ac-coupled input a second 10M
resistor could be connected in series with the inverting
input to virtually eliminate bias current error;
bypassing it would give minimal noise.
