6.2 DIGITAL SYSTEM COMPONENTS 311stops being incremented again. The value of the counter becomes the binary code for the analog
voltageVin, since the number of clock pulses passing through the control logic gate for a timet
is proportional to the analog signalVin. Dual-ramp A/D converters can provide accuracy at low
cost, even though the process is slow because a double clock pulse count is an inherent part of
the process.
Memory
For a digital computer which stores both programs and data, memory can be divided into three
types:random-access memory, mass storage,andarchival storage.Random-access memory
includes read-and-write memory (RAM), read-only memory (ROM), programmable read-only
memory (PROM), and erasable programmable read-only memory (EPROM), in which any
memory location can be accessed in about the same time. The time required to access data
in a mass-storage device is relative to its location in the device. Mass storage, such as magnetic-
disk memory, has a relatively large storage capacity and is lower in cost per bit than random-
access memory. Archival storage, such as magnetic tape, is long-term storage with a very large
capacity, but with a very slow access time, and may need user intervention for access by the
system.
Figure 6.2.15(a) Block diagram
of dual-ramp (dual-slope) A/D con-
verter.(b)Output of ramp generator
in dual-ramp A/D converter.