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AD9705 with current mirror load

Within my current research project I'm using an AD9705 DAC loaded with a wilson current mirror. The output load of the current mirror is a 120 Ohm resistor. I've set the full scale current of the DAC to 2mA.

My problem is now, that at the moment when the current switches from 0 to 2mA the settling time is far too long (about 65ns) and I notice that the bits switch one after the other (MSB first, then second most significant bit, and so on). On each current level (for each bit) the output stays for about 10ns.

As I do not know anything about the internal circuitry of the DAC I cannot explain this behaviour.

As a further explanation I've attached a jpg showing the measured output (at the load of the current mirror = 120Ohm).

Please, can anyone help me to fix that problem?!

Thank you very much.


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