In an integrating A/D converter, first and second reference voltage inputs (18, 20) alternatingly connect through a reference voltage switch (16, 16′) via a first reference resistor (Rref) to an inverting input (122) of an integrator (12). A comparator (22) connected downstream of the integrator (12) compares a test voltage applied to its test voltage input (221) with a comparator reference voltage applied to its reference voltage input (222). This input (221) is connected to- the output (126) of the integrator (12). A control device (40) actuates the first reference voltage switch (16, 16′) in a pulsed manner and measures the time intervals between the individual switching processes. An inverter (24) inverting a measuring voltage (UM) and a first heating resistor (RMH) coupled thermally with a measuring resistor (RM), are connected in series between the measuring voltage input (14) and the output of the first reference voltage switch (16, 16′).