function chamadaAjax(url, funcaoCallback){
   var xmlHttp;
   try  {
     // Firefox, Opera 8.0+, Safari
     xmlHttp=new XMLHttpRequest();
   }catch (e) {
      // Internet Explorer
      try{
         xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
      } catch (e) {
         try {
            xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
         } catch (e) {
            alert("Browser não suporta AJAX!");
            return false;
         }
      }
   }
   xmlHttp.onreadystatechange=function()
   {
       if(xmlHttp.readyState==4){
          // Get the data from the server's response
	  funcaoCallback(xmlHttp.responseText);
       }
   }
  xmlHttp.open("GET",url,true);
  xmlHttp.send(null);
}