Determine latin characters in cells

Named Ranges are always treated as arrays by Excel

Formula:

=INDEX(COLUMN(Sheet1!$A:$Z)+64,,)

So COLUMN(Sheet1!$A:$Z) is effectively {1,2,3......23,24,25,26}
By adding 64 we get {65,66,67......87,88,89,90} the Ascii codes for A-Z.

Similarly by adding 96 we get the codes for a-z.

Notice that we use COLUMN() not COLUMNS()
COLUMNS() returns a single value, in this case it would be 26.

Be careful where you use this, if any columns in the range A:Z are deleted the named range will change.
To minimize the risk of this happening we might refer to a higher range of columns
e.g.

Formula:

=INDEX(COLUMN($CW:$DV)-100+64,,)

Equally we might use

Formula:

=INDEX(ROW($1:$26)+64,,)

See if this workbook helps.