white zpass in cycles

hi all…
i try to render many times an d searched in the web for hours but i can´t find the way to produce a z pass in cycles. always white image an i play with contrast and brightness to be sure there is no unapreciable grey pixels.
any advice?

pd sorry about my poor english

Typically, you have to map values.

Make numbers up, find range to fit your scene.

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thanks you very much :), its working now.
i think i missing something .
¿ must i assume, that i need to re render the entire sequece to get a separated z pass for after effects compositing pruposes?

ok i figured out
i have to
1 make a test render,
2 then adjust z pass map values ,
3 finally set up an output node to all passes i need, everything before i render the entire sequence
4 render the entire sequence
5 i will find my passes separated i my output folder
am i right?