11.1 Introduction
Sample and hold circuits is used to sample an analog signal and to store its value for some length of time (for digital code conversion). It is heavily used in data converters. Sample-and-hold are also referred to as track-and-hold circuits. A few important performance parameters for sample-and-hold circuits:
1.Sampling pedestal or hold step: occurs each time it goes from sample mode to hold mode and there is always a small error in the voltage being held that makes it different from the input voltage at the time of sampling. This error should be small and even more importantly be signal independent to avoid nonlinear distortion.
2.Another measure is called droop rate in hold mode, which characterizes a slow change in output voltage in hold mode.
3.Aperture jitter or uncertainty, which is a result of effective sampling time changing from cycle to cycle. When high-speed input signals are sampled, it causes the held voltage to be significantly different from the ideal held voltage.
4.The speed. When OpAmps are used, its 3dB bandwidth and slew rate should be maximized for high speed operation at the price of power consumption.
5.Other performance metrics, such as dynamic range, linearity, gain and offset error are important too.