The most likely reason is that the onboard setpoint trimpot on the PLD is not zeroed, adding a DC bias voltage to the setpoint. The EXT SET input sums with the SET trimpot. To fix this, turn the SET trimpot completely counterclockwise (CCW) to zero it out.

Another possible reason is that the PLD current source is compliance voltage limited. For more information on limiting compliance voltage, see the Related FAQ listed below.