SN74LS138: 74LS138 G1 Enable pin - pulldown recommendation?

Bob Shaffer

Part Number: SN74LS138

Tool/software:

We have an SN74LS138 device, powered by 5V, with individual 10K pulldown resistors to ground on the G1 (pin6) and A/B/C (pins 1,2,3) inputs. When I measure the G1 input, without any signal driving it, it measures 1.05 volts (above the specified 0.8 volt VIL spec). What is the recommended pulldown resistor value on the G1 input to ensure the device outputs are disabled in the event the G1 input is not being driven (e.g. power up/down)?

Also, the SN74LS138 datasheet's recommended operating conditions specify a 2 volt VIH minimum, but no maximum. The absolute max input voltage is 7 volts. Is 5V (the rail) the recommended operating maximum VIH for these inputs with VCC = 5V?

Thank you, Bob

5 months ago

0 Malcolm Lyn 5 months ago

TI__Mastermind 19250 points

Hi Bob,

I would use anywhere from 1k-5k for a pull-down resistor.

It doesn't look like there's a positive clamp diode, but given nothing else by the datasheet I'd say that the maximum recommended input voltage is up to VCC for these devices.

Best,

Malcolm

0 Clemens Ladisch 5 months ago

Guru 277980 points

The specified input current can be as large as 0.4 mA. So to stay below 0.8 V, the pull-down must be smaller than 2 kΩ.

Logic

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SN74LS138: 74LS138 G1 Enable pin - pulldown recommendation?