What is the right way to use PDO directly in CodeIgniter 3?

I prefer to use PDO rather than codeigniter provided db class. i want to execute fetchAll(), prepare() statements which is not supported in db class provided by codeigniter

If i want to use PDO directly , should i place $db = new PDO($dsn,$username,$password); in every model class ? .

Is that bad ? What is the right way?

i am a newbie .