Limitations of comparators

Abstract

In most A/D converters the input signal is applied to an amplitude-limiting circuit. This amplitude-limiting circuit can be a differential amplifier stage or the input stage of a comparator. The amplitude of the input signal is usually much larger than the linear range of the input amplifier stages. In this way a limitation of signals for the individual amplifier stages occurs. This amplitude limitation results in variations of the delay times of the zero crossings of the differential stages. This can be explained in the following way. When a sine wave is applied at the input of the A/D converter, it looks as if this sine wave is cut into pieces with a variable slope. This slope depends on the level at which the input signal is equal to a reference voltage level. The variable slope introduces a variable delay of the zero crossing of the output signal. These delays are accordingly signal-slope-dependent. As a result, a nonlinear distortion of the input signal occurs while it travels through the A/D converter system. The moments at which ideal amplifiers would show zero crossings are shifted in time. These time shifts result in errors in the output code of the A/D converter. This delay variation is caused by the frequency limitation which is always present in practical amplifier stages. This distortion occurs in particular in bipolar amplifier stages that have a limited linear range. MOS amplifier stages have a larger linear range because of the much higher threshold voltage of the individual devices with respect to a bipolar transistor. A simple calculation and design model will be presented in this chapter. For additional information see [59].