impedance bandwidth of current source

[fu_zg]

impedance bandwidth

Hi guys,
I'm designing a current steering dac, which need a large impedance current source in wide bandwidth(tens of meg), when using a first stage cascode, the impedance is enough,but the bandwidth is too small, who can kindly tell me why?

Thanks

fu

 

[mickey0908]

Because the parasitical capacitance.
refer to "SFDR-BANDWITH LIMITATIONS FOR HIGH SPEED HIGH RESOLUTION CURRENT STEERING COMS D/A CONVERTERS"

BTW: I have a question , there is a big difference output impandence when use difference static bias current to simulation the output impandence?

 

[FvM]

By design, usual high speed current output DACs are intended for a low impedance load, e. g. 50 or 100 ohms. They never achieve their bandwidth with a high impedance load, apart from the fact, that the output compliance doesn't allow this operation. High (DC) output impedance is required anyway for high accuracy, but the output capacitance simply limits AC output impedance. 5 pF would be a usual amount.

So, did you compare our results with major manufacturers high speed DACs?. Did you achieve sufficient bandwidth with a meaningful load impedance?