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DAC088S085: DAC088S085 driving large capacitance

Carl Metelko
Prodigy 60 points

Part Number: DAC088S085

Hi,

     I have been caught out by the maximum capacitance load of 1500pF on the DAC088S085.  I need the DAC088S085  to charge

at least a 1uF capacitor possibly up to 10uF.  To keep the DAC088S085 stable  I am intending to charge the capacitor through a resistor.

Unfortunately I can't use the ibis model so don't know how large the resistor needs to be. (NB The time constant is not an issue).

How large a resistor do I require for a 10uF capacitor.

Thanks,

             Carl Metelko

  • 0
    TI__Genius 14220 points

    Hi Carl,

    Thank you for your query. If you are not concerned about the time constant, you can use a large resistor like 1k in series between the DAC output and the load capacitor. The trade-off comes only when you are concerned about overall bandwidth and DC voltage drop across the resistor. You can look at the following TI design that details out the method to decide the R-iso:

    However, because in this case, the amplifier is inside the DAC, there are few unknowns like open-loop output impedance, unity gain bandwidth etc. and there is no SPICE model for this device as well. If you are really looking for an optimized R-iso while keeping the DAC output stable, you can use the transient response method to analyze stability. The following E2E post provides the training material to do that. You can estimate phase margin from percentage overshoot.

    Hope I have answered your question.

    Regards,

    Uttam Sahu

    Applications Engineer, Precision DACs