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DAC8811 - Shift register contents at power-on?

Josh Purviance
Prodigy 10 points

Other Parts Discussed in Thread: DAC8811

I am curious about what contents of the shift register will be initially when the DAC8811 chip powers on.  The datasheet doesn't mention what its initial contents are.  It only states that the initial contents of the DAC register are filled with zeros, but says nothing about initial shift register contents.

I ask because when our product powers on and our microcontroller initially pulls the CS lines high during an initialization routine, this latches the initial contents of the shift register into the DAC register (consistent with the datasheet).  However, it is quite awhile (~100ms) before we can provide a new value via SPI into the shift register.  During this time, the output (Iout) seems to vary wildly within the Vref range.

Thanks,

-Josh

  • 0
    TI__Genius 10615 points

    Hi Josh,

    Josh Purviance said:
    It only states that the initial contents of the DAC register are filled with zeros, but says nothing about initial shift register contents.

    The initial shift registers are also filled with zeros. However both the shift and DAC registers are filled with zeros during the power-on, on most of our devices usually they are set by the time the supply reaches ~2 V. This is because the chip has an OTP (one-time programmable) register that is responsible for setting the DAC to its default state during power-on. It could be possible that this OTP is not triggering early enough.

    Josh Purviance said:
    During this time, the output (Iout) seems to vary wildly within the Vref range.

    Do you think you could provide me with a scope capture of this? I want to verify whether this is an expected power-on glitch, or another issue that we should take care of. MDACs usually don't display a large power-on glitch because they don't have a buffer. The usual suspect is either the output buffer or in this case the trans-impedance amplifier at the output.