DataGrid ItemTemplate TextBox Event Textchanged

[Cosi]

I have a datagrid with two ItemTemplate TextBox:
...
..
<asp:BoundColumn ReadOnly="true" DataField="MutDatum"
HeaderText="MutDatum" DataFormatString="{0:MM-dd-yyyy}"></
asp:BoundColumn>

<asp:TemplateColumn HeaderText="AT">
<ItemTemplate>
<asp:TextBox id="txtAT" runat="server"
AutoPostBack="True"
ontextchanged="txtAT_TextChanged" >
</asp:TextBox>
</ItemTemplate>
</asp:TemplateColumn>
<asp:TemplateColumn HeaderText="AHVLohn_neu">
<ItemTemplate>
<asp:TextBox id="txtBT" runat="server"
ontextchanged="txtBT_TextChanged" AutoPostBack="True">
</asp:TextBox>
</ItemTemplate>
...
When input value in the TextBox "txtAT" the TextChanged-Event whos
fired.
I Validate the value and then I wont to get back in the grid to the
next TextBox, but the focus from the tab ist lost.
How i can get the focus from the next ItemTemplate-TextBox?

Thanks many for helb