Layout design to minimize voltage-dependent variation on input capacitance of an analog ESD protection circuit

A design model to find the optimized device dimensions and layout spacings on the input ESD clamp devices is developed in this work to keep the total input capacitance almost constant, even if the analog signal has a varying input voltage. An analog ESD protection circuit has been designed to solve ESD protection challenge on the analog pins for high-frequency applications. The device dimension (W/L) of ESD protection device connected to the I/O pad can be reduced to only 50 μm/0.5 μm in a 0.35-μm silicided CMOS process, but it can sustain HBM (MM) ESD level up to 6 kV (400 V). With such a smaller device dimension, the input capacitance of this analog ESD protection circuit can be significantly reduced to only ∼0.4 pF for high-frequency applications. This input capacitance can be further reduced if the ESD protection devices are designed with smaller device dimensions. Moreover, by using the optimized layout design to draw the layout of ESD protection NMOS and PMOS devices, the voltage-dependent variation on input capacitance of this analog ESD protection circuit can be kept below 1% under an input voltage swing of 1 V. With such almost constant input capacitance, the nonlinear distortion causing by on-chip ESD protection circuit can be minimized for high-precision applications.

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