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BUF634A: IV conversion of current DAC using composite amplifier

Intellectual 830 points

Replies: 5

Views: 210

Part Number: BUF634A

Hello

  I am designing a IV stage (current output DAC) using FET opamp (opa1656 ) and I am thinking about using a buf634a as a composite amplifer . I am planing on having the bandwith if the BUF634A at 100Mhz to make sure the circuit is stable (2x bandwith of the first opamp)

  Would you say the above offers any benefits for IV conversion ? (vs single fet opamp)

5 Replies

  • Well, you are adding the delay of the BUF634A inside the loop which is a manageable stability issue, but maybe there is an op amp with adequate output current to begin with -what do you think you need in terms of output current or min loading? 

    What DAC are you using - most are complementary current output where using both outputs will improve HD2 if you care. 

    Michael Steffes

  • In reply to Michael Steffes:

    Its not about current but I am looking at slew and settling time, In a delta sigma DAC , you have very fast changes on current (steps) so looking at a composite opamp that will track those

  • In reply to user5327551:

    So the BUF634 is a unity gain voltage buffer - it cannot settle any faster than the core op amp - normally, composites with it are used for higher output current. You need something else I think. Only desired specs and source info can move this forward. 

    In the many delta sigma DAC designs I have done, here is an example using the THS4561 FDA implimented with a MFB wrapped around it - not necessarily what you want, but the point is there are a lot of options, 

    Michael Steffes

  • In reply to Michael Steffes:

    Hello Michael

       I am using a delta sigma DAC es9039q2m with a 2.4ma differential current output .

    I am already using the OPA1612 for IV stage , but I am trying to make a better IV stage

        In order to do that , I am planing to use a FET opamp opa1656 (that has excellent noise for a FET)  but I want to beef up significantly the slew rate (at least 100V/us bandwith (at least 100Mhz)  and settling time  (300ns or better at 0.01%)  at  . So I was thinking to use a composite amplifier . (of course THD+N is very important)

       Any suggestions ?

  • In reply to user5327551:

    Well not enough to go on yet, but couple of points, 

    1. What maximum step size

    2. Settling is a pulse response thing, HD is a sinusoidal test - doing both? 

    3. For best settling, stay out of slew limiting - whether a part slew limits is a small signal response shape issue with desired output step size. By way of preview, the peak dV/dT on a 2nd order response shape is approx 2.85*(Vstep) *F-3dB (and this is including any peaking effects, applies roughly from 0.7<Q<4).

    Tried to dispel some of the slew rate confusion in lower speed parts here - need to finish the higher speed slew rate article in the next couple of days, - so off to that, good luck with your questions. This one deals with transitioning from a largely slew limited step to a partially slew limited step for 1st order responses (but does add the effect of a real higher pole as well).. If you allow the edge to slew limit, you have transiently opened the loop requiring it to close again to find a final value - extending to the settling an uncertain amount. Stay linear if you can is the moral to this story.  

    https://www.planetanalog.com/separating-linear-from-slew-limited-performance-in-amplifiers-part-1-of-2-the-signal-sped-up-insight-14/

    Michael Steffes

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