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MSP430F6438 DAC output gain

Amardeep Manak
Prodigy 30 points
Other Parts Discussed in Thread: MSP430F6438

Dear All

I am using MSP430F6438 internal DAC, and have configured the Control registers for 3x output gain. The reference voltage is derived from REF module 2.5 V. 

I expect the maximum output from the DAC when i write 0xFFF in Data registers as 7.5 V.

But what ever may be the settings in DAC12IR and DAC12OG. The range of output from the DAC is always 0-Vref.

Somehow the multiplier seems to be not working.

  • 0
    Expert 2665 points

    It seems you misunderstood the whole concept of DAC gain. The max value you can get on DAC output is, according to spec = AVcc = 3.6V. This is when your Vref is also = AVcc. So basically possible output analog value from DAC can be in a range of 0 to Vref Volts no matter whether your gain is 1x, 2x or 3x.

    If you need 7.5V, you'll have to connect external buffer OpAmp to the output of DAC, and the power supply for this OpAmp should be at least 7.7...8V. There is no other way to do it.

    Now about DAC gain. This feature allows the user to control the dynamic range of the DAC. It limits DAC digital input full span. This means, for example, if gain = 3x, than in order to have max analog value (which is Vref) on DAC output you'll need to put to the DAC input just 1/3 of its max digital value. If you put a bigger digital value, DAC output will still limit its output analog value to Vref.

  • 0 in reply to Serge A
    Prodigy 30 points

    Hi SergeA

    I am observing the same behavior. Thanks for the prompt reply.

  • 0 in reply to Serge A
    Guru 227245 points

    Serge, your explanation isn’t correct.

    The maximum DAC output voltage is AVcc. However, the programming range is 0 to Vref.
    If you use GAIN=2 and Vref = 2.5V, and AVcc is 3.6V, then your maximum output is 3.6V and the DAC range can be 0 to 2950 (instead of 4095) and therefore 1.22mV resolution (instead of 0.61mV resolution for a range of 0..2.5V)
    With VRef = 1.5V and GAIN=2, the DAC output can go up to 3V. And with GAIN=3 and Vref=1.5V, output can go up to 3.6V again, but with 0..3276 range (1.1mV resolution).
    The DAC doesn’t support AVcc as ‘reference’. Mainly because it is too unstable to be useful when drawing output current. But with the gain, you can combine the precision of the reference with the full AVcc voltage range.

  • 0 in reply to Jens-Michael Gross
    Expert 2665 points

    Hi Jens-Michael,

    Thank you for correction and explanation. I never thought that DAC output can have higher voltage than Vref, probably because I always prefer to use 1x gain with full span DAC digital input capability. It is much more convenient to use 0.20$ external OpAmp where any gain can be specified. In fact, in many cases one has to use it anyway as a buffer/filter between a low-current DAC output and the rest of the system (speaker, servo, etc.)

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