11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 1 of 23

A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End

Achieving Sub-ppm Sensitivity

Xiahan Zhou, Michael Sveiven, Drew A. Hall

University of California, San Diego, La Jolla, CA, USA

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 2 of 23

Motivation: Point-of-Care (PoC) Devices

NEED: Accurate and fast diagnostic tests

Centralized Laboratory• Bulky instruments• Professional personnel required• Long turnaround time

Point-of-Care Devices• Small and portable• Easy operation• Near instant results

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

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Magnetic Immunoassay

Magnetic sensorHuman biological samples intrinsically lack magnetic background high sensitivity

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 4 of 23

Giant Magnetoresistive (GMR) Sensor

Large baseline-to-signal ratio precludes high sensitivity detection

Problem: Wide dynamic range AFE is required to accommodate large R0/Rsig

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 5 of 23

Conventional MR AFE Architectures

Problem:Sensor mismatch ∆R limits sensitivity

Problem:Fast transient signal requires fast-switching magnetic field and high speed ADC

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 6 of 23

System Architecture

Problems Proposed SolutionsLarge baseline/signal Reference sensorHigh speed ADC required Down-modulator in PGASensor mismatch HFIR in ADC

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 7 of 23

Double Modulation Scheme

Reference sensor rejects MR baseline

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 8 of 23

Analog Front-End

Built-in down modulator relaxes speed requirement on ADC

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 9 of 23

Sensor Mismatch

Sensor MR mismatch Residual baseline at DCSensor R0 mismatch Interference at fH

Sensor mismatch increases ADC dynamic range requirement

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 10 of 23

High Frequency Interference Rejection

ADC applies HP feedforward method to realize a fast-settling LPF function

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 11 of 23

High Frequency Interference Rejection

HFIR sampling technique rejects high frequency interference

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 12 of 23

PGA Implementation

• Switches remain closed after sensor selection for 100µs provide a low impedance path for fast settling

• Switches revert to duty-cycled mode afterwards provide large bias resistance for low noise

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 13 of 23

Fast Settling Duty-Cycle Resistor (DCR)

Fast settling DCR reduces the settling time by 40×

Measured transient waveforms

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 14 of 23

Incremental ∆Σ ADC

OSR: 10,000BW: 100 Hz

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 15 of 23

ADC Phase I

C4 samples signal at DCC6 samples OTA offset

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 16 of 23

ADC Phase II

φscr and φchop change at the beginning of Phase II Allow maximum time for settling

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 17 of 23

Die Photo and Power Breakdown

TSMC 180nm CMOS process

Total power: 1.39 mW

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 18 of 23

Measurement Results: ADC

HFIR improves ADC DR significantly when sensor mismatch > 2%

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 19 of 23

Measurement Results: System

The system achieves 0.98ppm (147μΩ) sensitivity with a readout time of 880ms

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 20 of 23

Measurement Results: BioAssay

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 21 of 23

Summary and Comparison

The chip achieves 22.7× faster readout time, >7.8× lower baseline, and 2.3× lower power than other GMR sensor-based designs

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 22 of 23

Conclusion

• Magnetic sensors are a promising candidate for PoC biosensing; however they suffer form large baseline/signal and sensor mismatch

• To address this we: Used reference sensors to reduce baseline Designed an integrated down-modulator to relax the ADC bandwidth

requirement Proposed a HFIR sampling technique to tolerate sensor mismatch Designed a fast settling duty-cycle resistor to improve readout time

• Result: A design that achieves sub-ppm sensitivity and tolerates 10% sensor mismatch

11.4: A Fast-Readout Mismatch-Insensitive Magnetoresistive Biosensor Front-End Achieving Sub-ppm Sensitivity

© 2019 IEEE International Solid-State Circuits Conference 23 of 23

Acknowledgement

• This work was supported in part by Qualcomm, Inc. and the National Science Foundation under grant No. ECCS-1454608

• The authors would like to thank MagArray, Inc. for providing the GMR sensors

Thank you for your attention