Differential ultra low noise amplifier for low frequency noise measurements

Almost all low noise voltage preamplifier suitable for application in the field of Low Frequency Noise Measurements (either commercially available or proposed in the literature) are single ended input amplifiers. This means that one end of the measuring port of the Device Under Test (DUT) must be connected to common ground. This may be a severe limitation in many interesting measurement configurations, such as the case in which Kelvin connections to the DUT must be employed. In this paper we propose a simple design of a fully differential input ultra low noise amplifier with noise performances, in term of equivalent input voltage noise, comparable to those of the best single ended input amplifiers for low frequency noise measurements reported in the literature. Indeed, the amplifier we propose is characterized by a voltage gain of 80 dB, in the bandwidth from a few tens of mHz up to a few kHz, and by an equivalent input voltage noise as low as 14 nV/√Hz (100 mHz), 2 nV/√Hz (1 Hz), 1.2 nV/√Hz (10 Hz) and 1 nV/√Hz (1 kHz). Such an amplifier can also be employed as a single ended input amplifier by simply shorting to ground one of the two differential input ports.