Ppf calculation unmatching in excel

Hello Friends,

can anyone help me in getting exact interest calcualtion of PPF that match with the bank and my mainained excel sheet.  For E.g. 

Opening Balance ( 01.04.2011 )                                                  = 26406/-

Addition during the year ( 05.05.2011 )                                        = 12000/-

Available Balance for Interest Calculation                                   = 38406/-

Interest Recd. from Bank On PPF Balance                                 =   1968/-

But As per my xls. Sheet Interest Comes to PPF Balance       =   3184/-

Pls. Correct me or my xls. sheet if found to be wrong.

Pls. find the attachement.

Regards

Sanjay