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Capacitive load stability of DAC6578

Feng Li2
Prodigy 80 points

The spec of  6578's capacitive load stability is 470pF@RL= infinite and 1000pF@RL=2kohm.  I understand this spec is upper limit. Is it right? If CL is 4.7uF and RL is 5.6k, how to keep stability? There isn't a series resistor before 4.7uF.

  • 0
    TI__Mastermind 43915 points

    Hi Feng,

    I did some testing in the past with the DACx578 family and capacitive loads. The internal structure of the DAC has an output amplifier which can drive the 470pF capacitive load directly and maintain stability. When a 2kohm  load resistor is added in parallel with the capacitive load, you can increase its capacitance up to 1000pF without having stability problems. Adding a series resistor, to the output of the DAC (Riso), will allow you increase the capacitive load beyond 1000pF without worrying about having the output oscillate. The cap load size, itself, will be dependent on the value of the Riso. A larger Riso can typically allow you to drive a larger cap load but keep in mind that the Riso is going to create a voltage drop creating an error.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 80 points

    I can't add a series resistor  bacause it will increase Rise-time of TDD mode. But there is a 4.7uF Cload in parrallel with 5.6K Rload. How to keep stability?

  • 0 in reply to Feng Li2
    TI__Mastermind 43915 points

    Feng,

    4.7uF is a fairly large capacitive load to drive directly with the DAC output buffer. You need to somehow isolate the capacitive load from the output buffer. This can either be done with an isolation resistor or using an external buffer amplifier that can drive 4.7uF directly.

    Regards,

    Tony Calabria