A.C.-coupled probe technique for very-low-frequency operation
The use of a junction-f.e.t. matched pair, functioning as a self-biased source follower, bootstrapped by the operational amplifier voltage follower it drives, facilitates the design of an a.c.-coupled oscilloscope probe with maximised input impedance. Consequently, a lower-cutoff frequency of less than 3 Hz is feasible with input coupling components that are not incompatible with current i.c. design practice.