Get Value of Row in Datatable c#

Dont use a foreach then. Use a 'for loop'. Your code is a bit messed up but you could do something like...

for (Int32 i = 0; i < dt_pattern.Rows.Count; i++)
{
    double yATmax = ToDouble(dt_pattern.Rows[i+1]["Ampl"].ToString()) + AT;
}

Note you would have to take into account during the last row there will be no 'i+1' so you will have to use an if statement to catch that.

for (int i=0; i<dt_pattern.Rows.Count; i++)
{
    DataRow dr = dt_pattern.Rows[i];
}

In the loop, you can now reference row i+1 (assuming there is an i+1)