A Track-and-Hold Circuit with Tunable Non-Linearity and a Calibration Loop for PAM-8 SerDes Receivers

In this brief, we propose a 60 GS/s high-linearity two-stage 8 x 8 time-interleaved track-and-hold circuit where it is possible to tune the static non-linearities of the second-stage buffer by applying a proper bias voltage. This allows us to maximize the static linearity of the buffer or introduce effects that counterbalance the non-linearities of other blocks of the analog front-end. To validate the proposed circuit, a prototype in TSMC 5 nm technology is designed and a linearity calibration loop is proposed for a Pulse Amplitude Modulation SerDes receiver. For the analog buffer, circuit-level simulations are performed in Cadence Virtuoso, while the calibration loop is simulated in MATLAB. The optimal bias voltage value can be found by modeling the track-and-hold linearity using a Taylor series and implementing the linearity calibration loop in MATLAB. By applying this result to the circuit-level simulation, we obtain a total harmonic distortion of over 50 dB, which matches with the maximum value achievable across the complete bias voltage control range. Lastly, the linearity of the system is also verified using a PAM-8 pseudorandom stream signal.