Analog signal to discrete time interval converter

Analog-signal-to-discrete-time-interval converter (written with or without hyphens; abbreviated as ASDTIC) is a specialized kind of an analog-to-digital converter, which converts the analog input signal (e.g. voltage or current) to quantized time intervals.

A/D converters The analog-to-digital converter is at the core of any dataacquisition system designed to transform data in the form of continuous analog variables into a discrete binary code suitable for digital processing. The ideal transfer curve for a 3-bit A/D converter is shown in Fig. 1.1.3.8 with the analog levels on the horizontal axis and the digital outputs which correspond to those input values on the vertical axis. Fig. 1.1.3.8: Conversion relationship for an ideal A/D converter - 24 - In order to select the best ADC for a given application, the application engineer should be familiar with the various types of converters available. The checklist in table 1.1.3.1 outlines the proper priorities for selecting an ADC. First and foremost, the required resolution of the converter must be determined. This will determine the number of recognizable quantization intervals in the ADC transfer function. Since all ADCs exhibit a fundamental quantization uncertainty of ±½LSB, the application engineer must choose an ADC with sufficient resolution to reduce this „digitizing noise“ to an acceptable low value. A useful rule of thumb is that each bit of resolution reduces quantization noise an additional 6 dB. Thus a 10-bit converter exhibits „best case“ quantization noise which is approximately 60 dB below full scale. Resolution however does not imply accuracy! Many A/D converters have been built with far higher resolution than accuracy (Reason e.g. temperature drift). Conversion speed is another important selection factor, since it defines the upper limit on system bandwidth. Generally speaking, the conversion speed requirements will dictate the type of converter selected. · Resolution · Accuracy - Initial (25o C) - Drift · Speed · Power requirements · Reference intern/extern · Interfacing (ser./parallel output) · Cost · Size Table 1.1.3.1: A/D selection features - 25 - The supply voltages required have to be available in the automation device. Applications that use a single ADC can be best satisfied by choosing an ADC with built-in reference. In systems where the reference must be variable, or if a master system reference is available, an ADC requiring an external reference can be used to advantage. Since most ADCs are interfaced to microprocessors, it logically folows, that interface logic should be included on-chip. Most newer ADCs include at least a three-state buffer for bus interface. However, high-speed data buses with significant activity during the conversion period may inject noise into the ADC and cause erratic or unstable output data. The solution to this problem (generally encountered only on 12-bit and higherresolution ADCs) is to use an external three-state buffer. Converter types (Sample) Type Features Integrating (Single slope, dual slope, multi slope) High accuracy Low speed Low cost Successive approximation High speed Accuracy at cost Tracking (Counter/Comparator) High speed in track Susceptible to noise Multicomparator "Flash" Highest speed High resolution expensive V/f Converter Fast responding Continous serial output Table 1.1.3.2: A/D converter types The list