A $16.5\text{nV}/\sqrt{}\text{Hz}$ Chopper Amplifier with AC-Coupled Technique for Ripple Reduction

This paper presents a chopper instrumentation amplifier for TMR sensor interface circuit. For high CMRR and DC gain, the amplifier employs a three-stage current-feedback topology with nested-Miller compensation. By chopping both the input and intermediate stages of the amplifier, a 10.3 mHz 1/f noise corner was achieved at an input-referred noise power spectral density (PSD) of l6.5 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{nV}/\sqrt{}\text{Hz}$</tex>. To reduce chopper ripple, the amplifier employs a continuous-time AC-coupled ripple reduction loop. Due to its continuous-time nature, the loop causes no noise folding to DC and hence offers improved noise performance over auto-zeroed amplifiers. The loop reduces chopper ripple by more than 60 dB, to levels below the amplifier's own input-referred noise. Furthermore, a maximum input-referred offset of 5 uV and a CMRR greater than 140 dB were obtained at a supply current of 231 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{A}$</tex> at 1.8 V.