What would the JS code be to add a character limit to a matrix constant sum table

Hi,
I'm trying to add a character limit to a matrix constant sum table. I'm using
Qualtrics.SurveyEngine.addOnload(function()
{
jQuery("#"+this.questionId+" .InputText").each(function () { 
  jQuery(this).on("input", function() { this.value = this.value.substr(0,15); });
});
});
But it doesnt work for Matrix tables.

Thanks
Mark