How should the unused N-input be treated on the CDCLVD1208 - pull-up or pull-down to ground?
I've seen conflicting guidance for both.
The CDCLVD1208 has two universal inputs, two pins on each input. If one of the inputs is unused, the datasheet recommends pulling both pins (P and N) to ground using a 1k resistor.
If an input is being driven by a single-ended CMOS signal, then only the P side will toggle and the N side will be connected to mid-rail (VIH-VIL)/2.
Actually lets verify how to correctly address both the p and n unused inputs.
The CDCLVD1208 spec sheet clearly states “For unused input, it is recommended to ground both input pins (INP, INN) using 1 kΩ resistors.”
An e-mail with Clancy Soehren stated “The factory recommends to pull the unused differential input of the CDCLVD1208 to a status level by having a pull-dn on the P-input and a pull-up on the N-input.” Is the data sheet wrong?
There is more than one way to address unused inputs for the CDCLVD12xx devices. The datasheet method will work okay. Clancy's suggestion will work as well as I have seen this used on other devices. Her suggestion to pull the inputs in opposite directions is helpful in the case where no input hysterisis is present to prevent noise from causing the input to toggle.
The CDCLVD12xx input buffers do have hysterisis built in so even floating inputs should not be a problem. The 1k pull down resistors are recommended just to be conservative. Additionally, only one input can be selected at a time using the IN_SEL pin, so activity on the unused input will not propogate to the output. This would not be the case for a dual buffer like the CDCLVD21xx.
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