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  Analog-to-digital converters

When measuring quantities of different nature, both constant and changing in time, one often tries to convert them into electric voltages and currents. Electric signals can be easily transmitted from remote sensors. There is an "arsenal" of standard devices and techniques for their processing. Previously, measured values were fixed by deflections of galvanometer needles, traces of paint on the tape of self-recorders, luminescence on the oscillograph screen. Modern measurement systems represent data, as a rule, in a digital form with the use of special transistor devices - analog-to-digital converters (ADC). The task of an analog-to-digital conversion consists in the transformation of the input voltage at the instant of measurement into a proportional value; if the voltage alternates then it is transformed into a discrete sequence of values.

Let us consider principle of ADC functioning with the example of the scheme shown in the Figure. It realizes so-called parallel approach: input voltage Uc is compared simultaneously to n reference (etalon) voltages which are formed with the aid of resistive potential divider Uref. The number of the reference voltages and an interval between their values are determined by the range of the measured values and required accuracy of the conversion - the number of binary digits of a value at the device output. Thus, one seven equidistant etalon voltages for the 3-bit representation which is realized in the considered example and allows to record eight different numbers including zero.

The measured voltage Uc is compared to the etalon levels with the aid of seven comparators ki whose input voltages take the values regarded equal to

  • 1 (binary) if the voltage at the input point (+) exceeds the voltage at the input point (-)
  • and 0 otherwise.
Thus, if the measured voltage does not fall outside the range [5U/2, 7U/2] then the comparators with numbers 1-3 are set into the state "1" and the comparators with numbers 4-7 - into the state "0". A special logical scheme (priority encoder) converts these states into a binary number z1z2z3 (011 in our example) or corresponding decimal number (3 in our example). If the voltage changes in time then one stores instantaneous value of the voltage at the input of comparators using selection-storage circuit shown by rectangles on the scheme and retain them constant during the measurement interval. Measurement instants are dictated by special tact signals; if they are periodic, then one gets an equidistant sequence of binary values (time series) at the output of the encoder. The described parallel approach to the realization of analog-to-digital conversion is simple but non-economical. Therefore, better approaches have been developed and are widely used, e.g., weighted and numerical.

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