Comparative Study of Trans-linear and Trans-impedance Readout Circuits for Optical Beam Deflection Sensors in Atomic Force Microscopy

The optical beam deflection sensor remains the most popular force detection method used in atomic force microscopy. With the recent development of short cantilevers, a means for measuring small deflections at high frequencies has become a challenge. Minimizing the noise level of the readout electronics without significantly limiting the detection bandwidth still remains a challenge. In this work, a recently proposed trans-linear readout circuit-based technique, in which necessary analog arithmetics are done in the current domain instead of the voltage domain, is compared to a more traditional trans-impedance readout circuit-based topology. Our developed trans-impedance readout circuit recorded a noise floor of 9.48 × 10⁻¹³ V² Hz⁻¹ compared to 1.41 × 10⁻¹¹ V² Hz⁻¹ for the trans-linear readout circuit. Also, the measured −3 dB bandwidth of 11 MHz for the transimpedance readout circuit was slightly higher than the 10 MHz for the trans-linear readout circuit. Trans-impedance readout circuits, with proper circuit design considerations and careful selection of electronic parts, still remain competitive for use in high-speed operations in atomic force microscopy.