I am trying to set up the AD8236 with +Vs=5V -Vs=0V Vref=2.5V but the ref pin doesnt work, it is always grounded.
When I make it double supply with +Vs=+5V and -Vs=-5V the ref pin works just fine.
I am using an ADA4004 to drive the ref pin as described by the datasheet.
I also noticed that the operation supply voltage of the AD4004 is +/- 5V to +/- 15V. Can you confirm that you're providing correct supply voltage and setup for the AD4004? Because if not, the problem might be on the AD4004 and not on the AD8235.