A low-offset small-area micropower chopper-stabilized instrumentation amplifier dedicated to sensor applications
Ong, Geok Teng
Date of Issue2012
School of Electrical and Electronic Engineering
This dissertation presents a new low-offset, small-area micropower chopper-stabilized instrumentation amplifier (IA) dedicated to sensor or sensor array interface circuitry in commonly-used sensors such as the Wheatstone bridge and two-input floating sensing element. The IA system comprises a differential difference amplifier (DDA) to sense floating small signals, a current source replica (CSR) circuitry which facilitates the pair matching layout for all critical pairs in DDA input stage such as the input transistors, current sources as well as the current mirror transistors for better matching characteristics, continuous-time injection-nulling switch (INS) with modified control clock choppers to reduce residual offset. The advantage of the proposed DDA with CSR facilitating the pair matching layout over conventional DDA design is that the pair matching layout is not limited to the input transistors pairs. An analysis on common-mode signals and differential common-mode signal in chopper-stabilized DDA is given. The proposed CSR serves dual functions: (1) to reduce the output ripple by generation the offset current nulling operation, (2) to improve input offset by enhancing the differential common-mode rejection (CMRRd) of the DDA, so that it offers better immunity to the differential-common mode signal that is not modulated by the chopping operation. Besides, high power supply rejection (PSR), low noise, micropower regulators are proposed in the work as the regulated power supply to power the proposed IA. The proposed regulators are based on op-amp-less architectural design which embodied the Brokaw bandgap regulator circuit in an additional feedback control loop so as to achieve high efficiency in terms of referenced PSR bandwidth per current. Another proposed regulator uses native composite power transistor and an on-chip native MOS capacitor in the pre-regulator to enhance broadband high PSR performance. Besides, a pseudo-resistor based low-pass filter is used to reduce the Brokaw’s voltage regulator circuit nosie. The IA system and high PSR regulators were fabricated using GLOBALFOUNDRIES 1.8V/3.3V CMOS 0.18µm process. The IA includes CSR circuit, biasing circuit and clock generator occupies an area of only 0.125mm2 whilst drawing 22µA at a supply of 1.8V. The IC has been experimentally tested. It achieves an input-referred dc offset of 1.78µV (mean+standard deviation) at the chopping frequency of 10kHz. For a closed-loop gain of 40.17dB, the output ripple is reduced by more than 3 times with respect to the reference INS chopper DDA without CSR circuit. At 10Hz, the DDA exhibits an open loop gain of 89dB, a phase margin of 54.1º at a unity gain bandwidth of 225kHz and a load of 220kΩ//56pF, a CMRR of 120dB and an input-referred noise of 62nV/√Hz. The proposed DDAs are dedicated to sensor circuit applications. They are compact whilst providing well-balanced performance metrics in area, offset, noise, power and bandwidth efficiency. Lastly, the proposed IA with the high PSR regulator has been tested with an emulated strain-gauge sensor to show its functionality as a data-acquisition block for small-signal amplification. The proposed IA system is able to achieve an input-referred noise of 3.76µVrms for 2kHz bandwidth, a SNR of 64.4dB and excellent power rejection capability of more than -46dB even at a high frequency of 500kHz. This indicates that the proposed IA system is suitable for use in micropower sensor applications that require low-power, low-noise, low-offset as well as small-area performance whist providing excellent immunity against the power supply fluctuation from external environmental influence. Therefore, the proposed strain gauge sensory system will be very useful for environmental monitoring system.
DRNTU::Engineering::Electrical and electronic engineering::Integrated circuits