A circuit portion for indicating a mutual capacitance between a first and second node is provided. The circuit portion comprises a switchable constant current source arrangement configured to supply a first current to the first node in a first direction or a second current to the first node in a second, opposite direction; a variable voltage source configured to output a voltage to the second node so as to hold the first node at a reference voltage; and a comparator arrangement configured to switch between said first and second directions of the constant current source when the voltage output by the variable voltage source reaches a lower threshold voltage or an upper threshold voltage and to output a signal in synchrony with said constant current direction switching. The signal is indicative of the mutual capacitance between the first and second nodes.