Gas Chromatography - Data Acquisition and Processing > The A/D Converter > Page 50

The output from most detectors ranges from zero to ten millivolts and the input range of many A/D converters is from zero to one volt. Thus, the instantaneous measurement of 0.2 mV from the detector must be scaled up by a factor of 100 to 0.2 volts, which is carried out by the scaling amplifier in the manner shown. 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 255. The digital data shown in figure 33 can be processed backward to demonstrate A/D procedure. It is seen that 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 32 is equivalent to 51 in decimal notation (32+16+2+1). It follows that the voltage that was converted must be . It should also be noted that because of the limitation of 8 "bits", the minimum discrimination that can be made between any two numbers is . It follows that 8 bit systems are rarely used today and contemporary A/D converters usually have at least 12 or 16 bit outputs.

The output from the A/D converter is sampled regularly by the computer and the curve relating this data to time will reconstitute the chromatogram. The precision of the chromatogram and any calculations made with the data will obviously depend on the frequency of sampling which is normally user selected.