The track and hold amplifier (TH or T/H) is the first amplifier in an A/D converter chain. Since the introduction of the first monolithic T/H amplifier in 1974, different architectures have been proposed and different circuit techniques have been employed to reduce the pedestal error without sacrificing speed and linearity of the system. For low speed A/D converters, closed loop techniques can improve the accuracy of the T/H by sacrificing speed. New applications like mm-wave wireless communications at 60 GHz, cognitive radios and cable television require a broadband A/D converter with bandwidths in excess of 850 MHz and moderate accuracy (7-10 bits) for digitizing the intermediate frequency (IF) signal and/or direct sampling. For these applications, open-loop T/H amplifiers can be used. However, the techniques introduced recently cannot necessarily be classified in the open loop or closed loop categories, as open loop circuit topologies can employ local feedback paths. Most of the techniques introduced in bipolar technologies, working at supply voltages in the order of 2.5-2.7V, cannot be transferred directly in modern CMOS processes as operation at about 1V is key for low power in digital.
U.S. Patent Publication 2009/0039923 discloses a track-and-hold circuit with low distortion; in particular, a track-and-hold circuit capable of tracking an analog input signal and holding a sampled voltage of the analog input signal at a sampling instant for processing by other circuitry, in response to a track signal that alternates with a hold signal. A first capacitor is provided, having a first terminal connected to a power supply terminal. Tracking circuitry operates when in an on state to apply through a resistor a tracking voltage to a second terminal of the first capacitor that corresponds to the voltage of the analog input signal, by applying the tracking voltage to a first terminal of the resistor, the second terminal of the resistor being connected to the second terminal of the first capacitor. A switch, responsive to the track signal and the hold signal, operates to switch the tracking circuitry to an on state in response to the track signal and to an off state in response to the hold signal, the time of change from the track signal to the hold signal comprising the sampling instant. A second capacitor is provided, having a first terminal connected to the first terminal of the resistor and having a second terminal connected to a power supply terminal. The second capacitor substantially reduces frequency-dependent harmonic distortion.
U.S. Patent Publication 2009/0072868 discloses a wideband track-and-hold amplifier which is provided in front of an analog-to-digital converter, receives and samples an analog signal, and transfers the sampled signal to the analog-to-digital converter, wherein an output load unit having an inductance component is connected to an input terminal of the analog-to-digital converter. Therefore, it is possible to compensate for a high capacitance component of an analog-to-digital converter, to increase the bandwidth of an output signal, and to improve system linearity.
U.S. Patent Publication 2007/0152863 discloses a digital calibration loop for an analog to digital converter; in particular, a method and apparatus to counter effects of an offset voltage by calibrating an analog-to-digital converter (ADC). A digital calibration loop minimizes the effects of offset voltage to improve ADC accuracy as well as provide a low-power, submicron-scale ADC. A calibration circuit senses an ADC output and adjusts a variable calibration voltage to counter the effects of the offset voltage. Reduction of the offset voltage effects increases the ADC accuracy.
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