A chopper current-feedback instrumentation amplifier with a 1mHz 1/ƒ noise corner and an AC-coupled ripple-reduction loop

In the precision mechatronics of wafer steppers, thermal expansion is an important source of error. To compensate for this, the temperature of critical mechanical components must be measured with high resolution (<1 muK), typically by using precision thermistor bridges. Furthermore, this resolution must be maintained over several minutes, which means that the amplifier used for bridge readout should have a 1/f noise corner of only a few mHz. Such noise performance is well beyond the capabilities of currently available CMOS amplifiers. This paper describes a chopper current-feedback instrumentation amplifier (CFIA) with a continuous-time (CT) ripple-reduction loop (RRL).The loop synchronously demodulates the amplifier´s output ripple, and then drives it to zero by canceling the offset of the input stage. Due to the CT nature of the loop, it does not suffer from noise folding. By using a three-stage nested-Miller topology, and chopping the 1st and 2nd stages, the amplifier achieves a 1/f noise corner of 1mHz, a noise density of 15nV/radicHz and an offset of less than 5 muV. Furthermore, the combination of chopping and the use of a current- feedback topology results in a DC CMRR of greater than 130dB.