Low-Level Measurements

This paper deals with the measurement of low values of electrical voltages and currents from sources with high internal resistance. The measured values are close to the theoretical limits of achievable sensitivity, which are determined by the magnitude of the Johnson´s thermal noise. Non-thermal noises are also generated in the circuit. The most significant noise after Johnson´s thermal noise is flicker noise with a frequency dependence of 1/f. Choosing an accurate operational amplifier with low input bias and noise is just as important as choosing precise passive components with low noise