Design of a CMOS Hall Effect Sensor for Auto Focus System Application
Date Issued
2015
Date
2015
Author(s)
Chen, Kuan-Chung
Abstract
Magnetic field sensor is widely used in commercial products and industry environment. Because Hall effect sensor can be integrated in CMOS process, it is applied in many systems. Although the cost of Hall effect sensor is lower, its sensitivity is poorer that its early applications is mostly for larger magnetic field strength. For example, sensing magnetic field strength or current around the fan of motor. With the advance of CMOS process and circuit design, Hall effect sensor begins to sense weaker magnetic field for consumer electronics. For instance, electronic compass in iPhone is implemented with Hall effect sensor. Hall effect sensing cell can be modeled as a Wheatstone bridge. Because of the process variation, the four resistor value of Wheatstone bridge will not be the same. This thesis presents spinning-current technique to solve this non-ideality. With chopper modulation technique, the offset caused by the transistor mismatch of the amplifier and the flicker noise of the transistor can be resolved. The architecture of the analog front end is capacitively-coupled instrumentation amplifier (CCIA); it achieves lower noise than that of a current-feedback instrumentation amplifier (CFIA) in the same power dissipation. The thesis also presents a T-capacitor network that saves 86% capacitor area of input and feedback capacitor. The offset from transistor mismatch will cause the triangle output ripple, so ac-coupled ripple reduction loop (RRL) is presented to overcome this problem. The magnetic field sensor is fabricated in TSMC 0.18-μm CMOS process. The core area is 0.13 mm2 and current dissipation is 160 μA from a 1.8-volt supply voltage. It achieves 174-kHz bandwidth and input-referred noise is 26.5 nV /√Hz. The linearity is better than 0.5% within ±400 mT magnetic field strength. The input-referred offset is smaller than 100 μT. Its resolution is 10 bits when applying ±100 mT magnetic field strength.
Subjects
Hall Effect Sensor
Low Power
Low Noise
Analog Front End
Instrumentation Amplifier
Auto Focus System
Type
thesis