After scaling the signal is be digitized by the A/D converter. The subject of A/D converters is well discussed in the Data Acquisition and Conversion Handbook [4]. The overall system is shown diagramatically in figure 36. Detector output ranges from zero to ten millivolts and the input range most A/D from zero to one volt. Thus, a detector output of 0.2 mV, must be scaled by 100 to 0.2 volts by the scaling amplifier. Now the A/D converter changes the analog voltage to a digital number, the magnitude of which is determined by the number of "bits" that the computer employs in its calculations.
If, for example, eight bits are used, the largest decimal number will be 26. The digital data shown in figure 36 can be processed backward to demonstrate A/D procedure. The third and fourth most significant "bits" (which are counted from the far left) and the two least significant "bits" (which are counted from the far right) are at the five volt level (high), which as shown in figure 36 is equivalent to 51 in decimal notation (32+16+2+1).Thus, the voltage that was converted must be 
. Because of the limitation of 8 "bits", the minimum discrimination that can be made between any two numbers is 
. Thus, 8 bit systems are rarely used today and contemporary A/D converters have at least 12 or even 16 bit outputs. Nevertheless, the precision of any measurement, at whatever level, 50%, 5% or 0.5% is rarely better than 0.4%, e.g. 50 ± 0.2 %, 5.0 ± 0.02 % or 0.5 ± 0.002 %, so in realistic terms, 8 bits of data, though less impressive, is quite adequate. As a matter of interest 8 bit systems can be very inexpensive (similar to the cost of a hand calculator) and can be completely satisfactory for the vast majority of chromatographic analyses.
Transmission of the Data to the Computer
The digital form of the data must now be made available to the computer. There are two basic modes of signal transmission; serial transmission and parallel transmission. In the serial mode, the digital word (number) is sent to the computer one bit at a time. Now a binary counter provides a parallel output since each of the output bits has its own data output channel and the value of each output bit is simultaneously available. To use a serial transmission scheme, this parallel output must be put into serial form. One way to accomplish this is to use a Universal Asynchronous Receiver Transmitter (UAR/T). It is sufficient to say that the heart of the UAR/T is a shift register and the shift register is strobed by a signal from the computer that displaces the binary number, bit by bit, sequentially from the register to the computer.
