A microcontroller has a compact 8-bit processor and a differential input sigma-delta ADC (SDADC). In a low-cost pyroelectric sensor motion detector application, a sensor output signal is supplied onto a second differential input of the SDADC. A first programmable internal reference voltage source supplies VREF1 via an internal signal path onto a first differential input of the SDADC. A second programmable internal reference voltage source supplies VREF2 onto a reference voltage input of the SDADC. VREF1 sets the center of the SDADC input sample window, thereby avoiding the need to provide an external AC blocking capacitor. VREF2 sets the size of the window. Proper window sizing and sample averaging and the high-resolution SDADC obviate the need for input signal amplification. Throughput requirements on the 8-bit processor are reduced by providing a hardware averager and associated DMA controller, thereby making the overall solution a low-cost, noise-insensitive, solution.