7.2.3 Application Curves

Figure 7-2. Application Scope Capture

7.3 Power Supply Recommendations

The power supply can be any voltage between the MIN and MAX supply voltage rating located in the Section

5.3.

Each VCC terminal should have a good bypass capacitor to prevent power disturbance. For devices with a

single supply, a 0.1 μF capacitor is recommended. If there are multiple VCC terminals then 0.01 μF or 0.022 μF

capacitors are recommended for each power terminal. It is ok to parallel multiple bypass capacitors to reject

different frequencies of noise. 0.1 μF and 1.0 μF capacitors are commonly used in parallel. The bypass capacitor

should be installed as close to the power terminal as possible for the best results.

7.4 Layout

7.4.1 Layout Guidelines

When using multiple bit logic devices, inputs should not float. In many cases, functions or parts of functions of

digital logic devices are unused. Such examples are when only two inputs of a triple-input AND gate are used,

or only 3 of the 4-buffer gates are used. Such input pins should not be left unconnected because the undefined

voltages at the outside connections result in undefined operational states.

Specified in Figure 7-3 are rules that must be observed under all circumstances. All unused inputs of digital logic

devices must be connected to a high or low bias to prevent them from floating. The logic level that should be

applied to any particular unused input depends on the function of the device. Generally they will be tied to GND

or VCC, whichever makes more sense or is more convenient. It is acceptable to float outputs unless the part

is a transceiver. If the transceiver has an output enable pin, it will disable the outputs section of the part when

asserted. This will not disable the input section of the I/Os so they also cannot float when disabled.

7.4.2 Layout Example

Vcc

Unused Input

Input

Output

Input

Unused Input

Output

Figure 7-3. Layout Diagram

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SN54AHCT240, SN74AHCT240

SCLS252P – OCTOBER 1995 – REVISED JANUARY 2025

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