An automatic document feeder is an electromechanical device consisting of a tray that holds a stack of documents and a complex system of rollers, pulleys, cables, sensors, etc., that feeds one sheet at a time to another device. The device to which sheets are fed may be a printer, a scanner, a fax machine, a photocopier, or anything else that processes one sheet of paper at a time.
An automatic document feeder is a very useful feature on any sheet-fed device. It relieves humans of the need to stand there waiting for the processing device to finish its work, then feeding one sheet at a time into the device. Compare a scanner with an automatic document feeder to a flatbed, single-sheet scanner. With the latter, you have to open a lid, position a document, close the lid, activate the scanner, wait for the scanner to finish scanning, open the lid, remove the document, insert another, and repeat. If you had an automatic document feeder you could simply drop a stack of documents into its tray, press a "start" button, and go do something else. When you returned, you would have a disk folder of scanned images and a neat stack of documents. Ta-da!
When shopping for a scanner with an automatic document feeder, match the feeder's capacity and speed to your needs. An automatic document feeder's capacity is simply the number of pages its tray or bin will hold. The capacity rating should specify the weight of the paper to which it refers, for that varies with the paper's thickness and directly impact's the automatic document feeder's capacity. If no paper weight is specified it's usually assumed to be 20 pound bond, the most common printing paper. The speed of an automatic document feeder is measured in pages per minute (ppm) and is of lesser importance than the overall speed of processing a single page; that is, the time it takes to feed a sheet and scan, fax, or do whatever the device does. Generally, the greater an automatic document feeder's speed and capacity the more it will cost or add to the cost of the device in which it is incorporated.
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