A Compact Low-Distortion Low-Power Instrumentation Amplifier

Abstract

A CMOS instrumentation amplifier based on a simple topology that comprises a double-input Gm-stage and a low-distortion class-AB output stage is presented. Sub-threshold design techniques are applied to attain high figures of differential-gain and rejection parameters. Analyses of input-referred noise and CMRR are comprehensively carried out and their dependence on design parameters determined. The prototype was fabricated in standard n-well CMOS process. For 5V-rail-to-rail supply and bias current of 100nA, stand-by consumption is only 16μW. Low-frequency parameters are ADM=86dB, CMRR=89.3dB, PSRR+=87dB, PSRR-=74dB. For a 6.5pF-damping capacitor, ΦM=73º and GBW=47KHz. The amplifier exhibits a THD of –64.5dB @100Hz for a 1Vpp-output swing. Input-noise spectral density is 5.2μV/ Hz @1Hz and 1.9μV/ Hz @10Hz, which gives an equivalent input-noise of 37.6μV, over 1Hz-200Hz bandwidth. This circuit may be employed for low-frequency, low-distortion signal processing, advantageously replacing the conventional 3-opamp approach for instrumentation amplifiers.