Edit in JSFiddle

  MathJax.Hub.Config({
    asciimath2jax: {
        delimiters: [['`','`'], ['$','$']]
    }
  });   
  
var myApp = angular.module('myApp',[]);


function MyCtrl($scope, $log) {
    
       var QUEUE = MathJax.Hub.queue;  // shorthand for the queue
   
    $scope.Update = function() {
        QUEUE.Push(["Text",MathJax.Hub.getAllJax("MathOutput")[0],"{"+ $scope.Expression+"}"]);
    }
    $scope.Expression = 'Original Expression: 4/2 * 9/3';

}
<div ng-controller="MyCtrl">
    <button ng-click="Update()">Update</button>

    <input ng-model="Expression" ng-change="Update()">
    <div id="MathOutput">
You typed: ${}$
    </div>
</div>

              

External resources loaded into this fiddle: