Variable Bandwidth Chopper Amplifier for Eddy-Current Non-Destructive Testing

This work focuses on an amplifier to be integrated in an Eddy Current (EC) Non-Destructive Testing (NDT) system presented in [1]. This system is designed for detection of superficial and internal flaws on conductive materials, using magnetoresistive (MR) sensors. The sensors can be biased with a DC+AC current to allow signal heterodyning. Biasing signal is analogically removed without the use of any digital circuit. Minimum working frequency is 1.5 Hz, allowing larger penetration depths. At such low frequencies, the amplifier presents large flicker (or 1/f) noise, which is mitigated using Chopper Stabilization [2]. The amplifier rejects DC and has a controllable pole frequency that can be tuned from 1.5 Hz to 1 Mhz. This tunability is achieved by implementing a novel tunable pseudo-resistor, to reduce large settling times when changing between sensors. The amplifier has a 12 nV/root Hz at 10 Hz with flicker corner frequency at 10 kHz and a noise floor of 4 nV/root Hz. Simulations were performed in AMS 0.35 mu m CMOS technology.