A compact 5800 μm² process monitor circuit is demonstrated which can measure multiple threshold voltages (Vth) locally on a die. An accurate, process/voltage/temperature-independent current source is provided to measure Vth using the constant current method to an accuracy of σ=3.6mV. The output is digitized by a sigma-delta modulator with a conversion time of 2ms. The circuit enables efficient and compact in-die variation monitoring of the key process parameters and is thus useful for the high-volume characterization of integrated circuit products. The circuit contains its own reference voltage, and thus can be used for calibration during testing or in the field