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I have an analog control signal and an H-bridge IC. Since I can configure my signal to vary over the TTL voltage range, it seems temping to send it straight to the driver and hope that it will threshold it for me. What will this chip, and in general TTL components, do when they are hooked up to analog signals that sometimes wander close to the high/low boundary?

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    \$\begingroup\$ What's wrong with using a 74LS/HCT14? \$\endgroup\$ Commented Nov 24, 2017 at 22:58
  • \$\begingroup\$ Well, If I could get away without it... (Ideally, if I had to have one, the intermediate ADC would have no deadband so that I could inject noise/a triangle wave and get easy PWM.) But you're right that's probably the way to go if this can't be done the silly way. \$\endgroup\$ Commented Nov 24, 2017 at 23:03

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The switching threshold voltage for a TTL input is undefined. It may vary between parts, and between makers (or vintage) for a given part.

If your analog signal is near the threshold voltage for the input, the part may oscillate randomly, considering the input as a High at one moment, Low at another.

If you really must send an analog signal to a digital (TTL or CMOS) input, you should really use a Schmidt Trigger input. The Schmidt Trigger has different thresholds for switching High and Low, so shouldn't oscillate for signals near or between the threshold voltages (but the threshold voltages are still undefined).

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  • \$\begingroup\$ +1 especially something like the L293 that has some larger currents flowing round it. You want those logic levels rock solid. Those kinds of drivers do not play well with others if the inputs dwell between thresholds. \$\endgroup\$
    – Trevor_G
    Commented Nov 24, 2017 at 23:24

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