Ultra-Low-Noise High-Input Impedance Amplifier for Low-Frequency Measurement Applications

The design of a low-frequency high-input-impedance amplifier having probably the lowest noise ever reported is presented. The amplifier's frequency range is from about 0.07 Hz to about 110 kHz at the -3-dB level. The equivalent input noise voltage spectral density is about 5.6, 1.4, 0.6, and 0.5 nV/radicHz at frequencies 0.1, 1, 10, and 1000 Hz, respectively. Gain of the amplifier is about 83 dB. Noise analysis is made for active-type, capacitive-type, and low impedance signal sources. The contribution from different noise sources in the amplifier and JFET to the overall noise is shown.