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I'm using the NI6509 and NI 6509e. According to the specifications, the minimum input voltage is 2V.
Does it mean, I can input a LVTTL signal (3.3V) ?
Is it recommended to use it like that? The alternative is to use a circuit to convert LVTTL to TTL, but, obviously, I'll be happy not to do it if not necessary.
I believe you shouldn't have any issues with using the NI-6509 to read a LVTTL signal. Just to be clear though, our device will not read a Logical high value until the voltage is above 2 Vs. I believe that since you have about 1.3Vs of clearance from this threshold you shouldn't run into any issues.
If you do notice issues with this, converting this to a TTL would be a good solution as well.
Jim St National Instruments RF Product Support Engineer