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DAC Output range

paddu1
Expert 1195 points
Other Parts Discussed in Thread: MSP430F6636, MSP430F4616

Hi,

We are using MSP430F6636's DAC to generate sine wave,
and there is some problem with the DAC output.

When the supply voltage(Vcc) is around 2.8V
the sine wave's min(bottom) level is almost near to the "0"
but when Vcc increases to 3.5V, the sine wave's bottom level
also increases to 1V (instead of staying near "0").

According to the datasheet's "DAC, Output Specifications"
the Output voltage range for the bottom level will be 0.1V at MAX.

Could you please let us know how to keep the bottom level near to "0"
even if there is any voltage(Vcc) fluctuaion.

Regards.
Paddu

  • 0
    Guru 227245 points

    The DAC can only source or sink 1mA. So it's possible that with higher supply voltage your attached circuitry leaks a current that the DAC cannot sink.

    Is the sine wave still a sine wave or is it distorted (squeezed)?

    paddu1 said:
    the Output voltage range for the bottom level will be 0.1V at MAX.

    For a load >3k and a capacitance < 100pF.

    For DAC values below 0.3V and above VCC-0.3V, the DAC has an output impedance of 150Ohm and more.

    I didn't ever use the DAC for producing a sine wave and not on this specific MSP, but when I used it, I had no problems reaching 0V to VCC output with VCC of 3.6V

  • 0 in reply to Jens-Michael Gross
    Expert 1195 points

    Thank you Jens,

    And I am sorry for the delay.
    Actually we are checking the output directly without any filters
    and the output is still a sine wave,it is not distorted.
    There was no such issues when we tried with the Internal reference(REF).
    But, the internal REF is MAX 2.5V and it is not sufficient,
    Vcc would be better.

    Regards.
    paddu

  • 0 in reply to paddu1
    Guru 227245 points

    paddu1 said:
    Actually we are checking the output directly without any filters

    I wasn't talkign about filters. it was about load. Even a simple scope probe usually has up to 20pF capacitance which loads the output.

    paddu1 said:
    and the output is still a sine wave,it is not distorted

    Hmmm... So teh DAC seems to no longer use GND as negative reference level.

    paddu1 said:
    the internal REF is MAX 2.5V and it is not sufficient

    AVCC is also available as reference voltage.

    Do you have any voltage on the VREF- input pin? It shouldn't matter (AVSS should be the GND voltage even with external reference), but who knows.

  • 0 in reply to Jens-Michael Gross
    Expert 1195 points

    Hello Jens,

    Thank you so much.
    Somehow we are able to manage with internal reference,
    so we have decided not to use the Vcc.

    Regards.
    Paddu.

  • 0 in reply to paddu1
    Guru 227245 points

    paddu1 said:
    Somehow we are able to manage with internal reference,

    Well, good if you found a workaround.

    It doesn't give an explanation for what you observed, and I really dislike unsolved riddles (they usually come back to you when you have even less time to solve them than before).
    Bbut if you got a solution that works for you, fine.

  • 0 in reply to Jens-Michael Gross
    Prodigy 30 points

    Hi,

    I am using the  MSP430F6636 DAC12 to generate 40 KHz ferquency in steps. and facing the problem of distorted wave form( glitches on edges) with DAC amplifier on highest setting(DAC12AMP_7) . while using same hardware circuit and MSP430F4616,there was not such problem.

    Please could you suggest the solution and reason for same.

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