Devices, systems, and methods for counting a quantity of fail-bits in a memory device using a VFC circuit are disclosed. The VFC circuit can be calibrated via an offset adjustment mechanism to compensate an internal mismatch. The VFC circuit can include a processing unit and a circuit coupled to the processing unit. The processing unit can receive a first signal representing a number of detected fail-bits in the memory array, process the first signal and an offset signal, and generate a second signal representing a quantity of fail-bits. The circuit can receive the first and second signals, determine a difference between the quantity of fail-bits represented by the second signal and the number of detected fail-bits represented by the first signal, adjust the offset signal according to the difference, and provide the offset signal to the processing unit.