Linearized Depth Buffer - always black output

Hi! There is article on the internet, talking 'bout how to linearize depth buffer. (I want always to have it full colors from 0 to 255)

http://www.mvps.org/directx/articles/linear_z/linearz.htm

The code sniper in there is:

 float4 vPos = mul(Input.Pos,worldViewProj);
 vPos.z = vPos.z * vPos.w / Far;
 Output.Pos = vPos;

I thought of Unity equivivalent as:

 Input vert (float4 vertex : POSITION) 
 {
 Input o;
 float4 oPos = mul(o.pos,UNITY_MATRIX_MVP);
 o.Z = oPos.z * oPos.w / _ProjectionParams.z;
 return o;
                 
                 
 }

But, I always get black output!

 void surf (Input IN, inout SurfaceOutput o) {
     float4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
     o.Albedo = IN.Z.r;
 
 }

Am I wrong somewhere? if I use 1f - o.Albedo = IN.Z.r, zbuffer is always (51,51,51) color. HMMMM.