Interface ASIC Design For TMR-effect-based Magnetic Sensors

Magnetic sensor is an important branch of sensors.Magnetic sensor detects magnetic signals or the physical quantities related to magnetism and transform them into electrical signals.There are many kinds of magnetic sensors and tunneling magnetic resistance sensor(TMR)is one of them.Compared with other magnetic sensors,TMR sensor has both advantages in high sensitivity and wide dynamic range.However,the output signal of TMR sensor is small and mostly in low frequency.Since the general detection circuits have the disadvantages of high offset voltage and high low-frequency noise,they are not competent for the detection of TMR sensor signal.So it is very important to design an ASIC for TMR sensor.In this paper,the interface ASIC includes three parts: instrumentation amplifier,curvature-compensated CMOS bandgap voltage reference and Sigma-Delta modulator.Instrumentation amplifier is used to receive weak signals from the TMR sensor,so the circuit should have the advantages of low noise,low offset and low distortion.This paper analyzes the source of circuit noise and offset voltage and then use chopper technology to cancel the 1/f noise and offset voltage.Since the offset and 1/f noise can cause high frequency ripples in the output,this paper also designs a ripple suppression circuit.Simulation shows that the amplifier's equivalent input noise is 16.17 nV/sqrt(Hz)with the corner frequency 1.89 mHz;the amplifier's bandwidth is 50 kHz;the amplifier's the harmonic distortion is 0.0017%;the amplifier's the static power consumption is only 277 ?A and the circuit can inhibit high frequency ripples.Curvature-compensated CMOS bandgap voltage reference is used to provide a stable reference voltage for the TMR sensor circuits.In this paper,the theory and noise performance of the curvature-compensated CMOS bandgap voltage reference is analyzed and chopper technology is used to reduce the low-frequency noise.The simulation results show that the temperature drift of the bandgap voltage reference is 1.957 ppm/?,the output voltage noise density of the designed reference is 956.6 nV / sqrt(Hz)@ 1 Hz,and the static power consumption is 82.1 ?A.Sigma-Delta modulator is the core of the Sigma-Delta ADC.In this paper,the stability and noise characteristics of the Sigma-Delta modulator are analyzed and the circuit-level Sigma-Delta modulator is designed.The simulation shows that the designed Sigma-Delta modulator has a noise density of-145 dB/ sqrt(Hz)and the modulator's signal bandwidth is larger than 50 kHz.At the last of this paper,the performance of the total interface circuit is simulated.The simulation results show that the total noise of the ASIC is about-120 dB/sqrt(Hz),which meets the requirement of high precision.