TIME RESOLUTION OF NMOS SAMPLING SWITCHES USED ON LOW-SWING SIGNALS

A number of recently reported CMOS line receivers and downconversion m ixers are based on sampling, A key component in these designs Is the N MOS sampling switch, It can sample a very high bandwidth signal, sever al GHz for a 0.8-mu m transistor, We present an expression for the ape rture time for an NMOS switch when the input has low swing, The switch can, under this condition, be modeled as a device that determines a w eighted average over time of the input signal. The weight function is derived. The aperture time function shows that the maximum theoretical time resolution for a switch in 0.8-mu m standard CMOS is 21 ps (simi lar to 48 Gb/s), SPICE simulations agree with the theory, Transient tw o-dimensional (2-D) device simulations do not contradict the predicted results, Experiments on a switch made in a 0.8-mu m standard CMOS pro cess show successful sampling of every thirty-second bit of a 5-Gb/s d ata stream.