SN74AS138A

The 54ALS138, 74ALS138A, and 4AS138 circuits are designed to be used in high-performance memory-decoding or data-routing applications requiring very short propagation delay times. In high-performance memory systems, this SN74AS138A decoder can be used to minimize the effects of system decoding. When employed with high-speed memories with a fast enable circuit, the delay times of this decoder and the enable time of the memory are usually less than the typical access time of the memory. This means that the effective system delay introduced by the Schottky-clamped system decoder is negligible.

The SN74AS138A conditions at the binary select inputs and the three enable inputs select one of eight input lines. Two active-low and one active-high enable inputs reduce the need for external gates or inverters when expanding. A 24-line decoder can be implemented without external inverters and a 32-line decoder requires only one inverter. An enable input can be used as a data input for demultiplexing applications.

The SN54ALS138 and SN54AS138 are characterized for operation over the full military temperature range of 55°C to 125°C. The SN74ALS138A and SN74AS138 are characterized for operation from 0°C to 70°C.