1. A circuit including an active filter having a plurality of first linear electronic elements arranged as a switchable array and at least one second linear electronic element, the first linear electronic elements having an electrical characteristic represented by a first design value and the second linear electronic element having an electrical characteristic represented by a second design value, the first and second linear electronic elements substantially determining a time constant of the active filter, and a tuning circuit for outputting a digital code n to the array of first linear electronic elements to adjust the time constant of the active filter approximately to a predefined value, the tuning circuit comprising: a first linear tuning element having an electrical characteristic represented by the first design value times a first predefined factor K C ; a second linear tuning element having an electrical characteristic represented by the second design value times a second predefined factor K R ; a signal generator configured to generate a signal indicative of a tuning time constant determined by the first and second linear tuning elements; a backward counter having a reset input configured to reset the backward counter upon receipt of a reset signal to an initial value x max that includes a relationship of the time constant of the active filter circuit and the tuning time constant; a clock input connected to receive a clock signal, an enable input connected to the signal generator to receive a signal for starting the counter with the initial value x max and stopping the counter in order to generate a count value x n, and a counter output configured to provide the count value x n ; and a decoder configured to convert the count value x n into a digital code n for switching the array of first linear electronic elements.