ENCODERS, DECODERS AND RECODERS

Adders.

Cliff Strahan, May 22, 2001

An Adder is a circuit that produces an output that is the sum of the inputs.

A half-adder is a single-bit adder that is only suitable for the LSB, as it does not have a carry-in function.

A full-adder is a single-bit adder that is suitable for any bit, as it has a carry-in function.

Adders in parallel can be used to make a multi-bit adder.

Adder circuits are binary adders. Decimal (BCD) addition requires a decimal adjust circuit.

Comparators.

A digital comparator compares two binary numbers and indicates when they are equal.

A digital comparator is implemented with logic gates, normally an ex-or / and gate array.

A digital comparator can be used to indicate when a counter has reached a pre-set count.

See Floyd p144.

An analog comparator compares two voltages and indicates which of them is the more positive. A logic 1 out indicates that the + input is more positive than the input.

An analog comparator can be implemented as an op amp without any feedback. The output is always saturated, either + or -.

An analog comparator is used in analog to digital converters (ADCs) to compare the unknown voltage to a reference voltage.

See op amps, Floyd p687.