A Differential Impedance Flow Cytometry Front-End with Baseline Current Cancellation

In this work, we present a high-performance analog front-end (AFE) circuit for impedance-based flow cytometry readout. The AFE is designed to interface to a three-electrode sensor topology using center electrode excitation and differential current output. To satisfy the needs of a differential high gain signal path, we propose a digitally tunable and calibrated cancellation current generation path to remove the baseline current injected into the transimpedance amplifier (TIA) stages. This prevents TIA saturation and allows for higher gain. Consequently, the AFE is more power efficient while maintaining better noise and interference rejection. The proposed circuit is designed and fabricated in a 180nm CMOS process. It covers an excitation frequency range of 0.5MHz to 10MHz and consumes 15.6mW during nominal operation. Digital calibration is implemented using an off-chip ADC and automated calibration algorithm. Measurement results show that at 1MHz excitation, the AFE achieves $1.7 \text{pA}/\sqrt{\text{Hz}}$ input-referred current noise density with floating inputs. The AFE achieves detection of 3um diameter particles in a microfluidic flow cell, demonstrating its performance and practicality for impedance flow cytometry.