Ultralow Noise Low Offset Chopper Amplifier for Induction Coil Sensor to Detect Geomagnetic Field of 1 mHz to 1 kHz

An induction coil sensor (ICS) is important for measuring low-frequency (DC-1kHz) geomagnetic field. The accuracy of the preamplifier is one key factor determining the performance of the sensor. But the preamplifier is susceptible to 1/ f noise, offset voltage and drift. In order to eliminate these influences, a preamplifier circuit with three amplifier stages based on chopper technology has been designed, and its performance has been tested. The results show that: 1) The 1/ f noise corner frequency is 3 mHz, the equivalent input voltage noise (EIVN) level of the circuit is [Formula: see text] and [Formula: see text]; 2) The equivalent input current noise (EICN) level of the circuit is [Formula: see text]; 3) The offset voltage is about 600 nV, and the time drift performance is excellent. In conclusion, the preamplifier circuit has characteristics of ultralow noise, low offset voltage and low time drift. It can effectively amplify low-frequency weak geomagnetic signals from 1 mHz to 1 kHz and provides excellent performance for low-frequency ICS.

A Low-Noise Chopper Amplifier with Offset and Low-Frequency Noise Compensation DC Servo Loop

The low-frequency and low-amplitude characteristics of neural signals poses challenges to neural signals recording. A low noise amplifier (LNA) plays an important role in the recording front-end. A chopper-stabilized analog front-end amplifier (FEA) for neural signal acquisition is presented in this paper. It solves the noise and offset interference caused by the servo loop in the chopper amplifier structure. The proposed FEA employs a switched-capacitor (SC) integrator with offset and low-frequency noise compensation. Moreover, a dc-blocking impedance is placed for ripple-rejection (RR), and a positive feedback loop is employed to increase input impedance. The proposed circuit is design in a 0.18-µm 1.8-V CMOS process. It achieves a bandwidth of up to 9 kHz for local field potential and action potential signals acquisition. The referred-to-input (RTI) noise is 0.72 µVrms in the 1 Hz~200 Hz frequency band and 3.46 µVrms in the 200 Hz~5 kHz frequency band. The noise effect factor is 0.43 (1 Hz~200 Hz) and 2.08 (200 Hz~5 kHz). CMRR higher than 87 dB and PSRR higher than 85 dB are achieved in the entire pass-band. It consumes a power of 3.96 µW/channel and occupies an area of 0.244 mm2/channel.