Codeigniter return Json from controller to dynamic table in view via Ajax request – Magento

I get data from Magento database and I need to return it as an Json object and add data to the table in view. I’m getting a html response, not a valid json.

This is the error message :-

DataTables warning: table id=order_list – Invalid JSON response. For
more information about this error, please see
http://datatables.net/tn/1

Here is my controller code:

public function prepareJson(){

    $orders = Mage::getModel('sales/order')->getCollection()->setOrder('created_at', 'DESC')->setPageSize(25);
    $total_records = $this->orderModObj->get_total();
    $tableData = array();

    $jsonObj =new stdClass();
    $jsonObj->sEcho = 5;
    $jsonObj->iTotalRecords = $total_records;
    $jsonObj->iTotalDisplayRecords = $total_records;

    foreach ($orders as $ordercus) {
        array_push($tableData, $ordercus);
    }
    $jsonObj->aaData = array($tableData);

    $tableJSON = json_encode($jsonObj);
    return $tableJSON;
}

Here is my view code

<table id="order_list" class="table table-bordered table-striped" data-page-length="25">

Here is the json structure

{
"sEcho": "5",
"iTotalRecords": 2708,
"iTotalDisplayRecords": 2708,
"aaData": [
    ["300000374", "16-11-2017", "16-11-2017", "title ", "charith m ", "<img src="http://mysite.lk/dif.gif" class="image" alt="profile picture">charith", "AUD 100.00", "<div class="processing">Processing</div>", "<a class="tb-edit" href="http://mysite.lk/view/300000374">View </a>"]

]
}

Here is the javascript code

$(document).ready(function() {

$('#order_list').DataTable( {

    "processing": true,
    "serverSide": true,

    url: BASE_URL + 'order/prepareJson',
    method: 'post',
    data: { get_param: 'value' },
    dataType: 'json'
    //"ajax": " "
} );
$.ajax({
    success: function (feedback) {
        //console.log("feedback", feedback);
        $("#order_list").append($(feedback));
        }
    });
});