Writing new record in a table upon the update of a cell or cell range

I receive automatic updates of a range of cells from an outside source (stock market trades). To accomplish this each cell has a formula in it like for example =RTD("esrtd",,"XLI","Volume"). How can I write a new record in a table every time I receive an update, basically creating a log of market activity? To simplify the question:

How can I make the code below run when the value in A1 is updated form the outside source? (It works if I manually type something in A1, but does not work when A1 is updated via a formula).

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
'YOUR CODE HERE
End If
End Sub

Assumes A1 is the cell