An analog to digital converter according to a multiple-ramp procedure for performing continuous integration of an input signal (9) applied thereto, comprising
- a summation circuit (1) having a first input connected to said input signal (9);
- a charge storage circuit (2) having an input connected to an output of said summation circuit (1),
- a comparator circuit (3), with a switching threshold, having an input connected to an output (13) of the charge storage circuit (2), and generating a comparator output signal (16);
- a logic circuit (5) evaluating said comparator output signal,
- at least two analog switching control lines (17) connected to respective outputs of said logic circuit (5);
- a clock oscillator (6) applying a clock signal (14) to an input of said logic circuit (5);
- and
- a switching circuit (7) applying, as an input signal to said charge storage circuit (2), at least one of a first reference signal (r1) and a second reference signal (r2), as determined by control inputs connected via said analog switching control lines (17) to said respective outputs of said logic circuit (5),
- wherein said comparator circuit (3) responds to an input signal (13) which crosses its switching threshold by producing a comparator output signal which triggers cessation of application of a currently applied one of said reference signals (r1,r2) and, at a predefined time with respect thereto, resumption of application of a different one of said reference signals (r1,r2), said triggering occurring in said logic circuit (5) at periodically recurrent time intervals at an edge of said clock oscillator output signal (14);
- said logic circuit (5) generates an output signal (12), corresponding values of said summation circuit input signal (9) and said logic circuit output signal (12) collectively defining a transfer function H(z) of said analog-to-digital converter;
- wherein, whenever at least one of said first and second reference signals (r1,r2) is zero, said transfer function H(z) has an Nth order, N.gtoreq.1, low-pass filter characteristic;
- `those instants when one of said reference signals is applied to said summation circuit (2) are defined as a "switched-on state" and said reference signal, and the duration of the switched-on state of the reference signal in a settled-down state and neglecting quantization noise, is a linear function of the input signal (9), and wherein a quantization noise transfer function
- H.sub.q (z)=1/(1+H(z))
- has a high-pass filter characteristic, thereby achieving greatest possible suppression of quantization noise in the frequency range of the input signal (9) being converted in said analog-to-digital converter.