Problem with php AJAX accents PHP MYSQL using PDO

I can not receive the JSON from the php because the data has an accent I leave the code: PHP:

 <?php  
$pdo=new PDO("mysql:dbname=salas;host=127.0.0.1","root","");
$statement=$pdo->prepare("SELECT * FROM departamentos");
$statement->execute();
if (!$statement){
    echo 'Error al ejecutar la consulta';
}else{
    $results = $statement->fetchAll(PDO::FETCH_ASSOC);


}
echo  json_encode($results, JSON_UNESCAPED_UNICODE);
?>

Javascript code

var departamentos = [];
tablerosR();
function tablerosR() {
    tableros = [];
    if (window.XMLHttpRequest) {
        xmlhttp = new XMLHttpRequest();
    } else {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            var tab = JSON.parse(this.responseText);
            for (let i = 0; i < tab.length; i++) {
                departamentos.push(tab[i]);
            }
            btn_trello();
        }
    };
    xmlhttp.open("GET", "php/departamentos.php?_=" + new Date().getTime(), true);// new Date().getTime() lo usamos porque en explorer estaba cacheando la consulta y 
                                                                                //así modificamos la url consiguiendo que sea otra consulta
    xmlhttp.send();
}