reducing glitch and charge injection
there are 256 resistors placed from vref to gnd and some switch behind of each res node,when one of switch is off and another switch is on the charge will be injected in the res string and the node voltage of res string will be changed ,through about 500ns the node voltage back to the initial value , this time is so long and the output voltage will be changed .this will produce the glitch.
maybe I should improve the response time of op(vref buffer)
maybe I should improve the value of res string.
maybe ..........