Converting string to json to html from PHP

I've got this use this ajax  and json to pass the value of a table to server side PHP:

$.ajax({
  type: "POST",
  url: "json1.php",
  dataType: 'html',
  data: { json_1 : JSON.stringify(newArray), json_2 : JSON.stringify(newArray1) ,'key' : fileKey},
  success: function(response){                           
       $('#output').html(response);                      }
  });

on the server side I have no problem, but returning the value I've got this error

Catchable fatal error: Object of class PHPExcel_Writer_Excel2007 could not be converted to string from blah blah blah

I can't figure out what is the problem. Maybe its the JSON cause its converting the html element to JSON, but I already decode it on the server side 

$jsonData1 = json_decode($jsonData_1); 
$jsonData2 = json_decode($jsonData_2);