Analog I/O refers to analog inputs (I) and analog outputs (O). The analog input on a data acquisition board or card is the input to an analog-to-digital (A/D) converter. The analog output on a data acquisition board or card is the output from a digital-to-analog (D/A) converter.

The analog I/O input function of the A/D converter typically allows input signals with a range from plus to minus 10 volts to be converted into a 12- to 16-digit representation of the input voltage.

For a typical unipolar A/D converter, the value assigned to each digit is predetermined by design. The first most significant digit [the first digit on the left] has an analog weight equivalent to 5 volts; the second most significant digit has a weight of 2.5 volts; the third has a weight of 1.25 volts. Each successive digit has a weight of one-half the previous digit or bit. Therefore, the 12th bit of a 12-bit A/D converter has a weight of 2.44 millivolts.

The 12-bit A/D converter above has digitized 8.75 volts into a digital output code of [111000000000]. To see that the code is equal to 8.75 volts, the weights of each digit are simply added together, 5 volts + 2.5 volts +1.25 volts = 8.75 volts. If the code were [111000000001], the input signal would be equal to: 8.75244 volts.

The analog I/O output function of the D/A acts exactly opposite of the A/D converter. A digital code of [111000000001] sent to a 12-bit D/A converter would result in an analog output of 8.75244 volts.

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