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MSP430F5358 DAC12: any information about internal diagram

Hardy Griech
Expert 2870 points

Hi,

in our design we had an issue with the DAC12: my hardware college thought that it would be great to put a capacitor (100nF) directly at the output of DAC1 (at P7.7), then put the signal into an 9.4k resistor and after that there is again a capacitor.  This all for EMC reasons.

Result was, that DAC12 draws an additional current of appr. 10mA(!) @ 3V.  This was because of oscillation due to the first capacitor.  Removing the capacitor showed the expected current draw.

So we both are wondering why this could happen, because our idea of the MSP430 internals is, that there is a simple amplifier at the output which drives the analog signal.

Is there any information about the DAC12 internals or an application note about (correctly) connecting the DAC12 to external circuitry?

Thanks

Hardy

  • 0
    TI__Mastermind 49692 points

    Hi Hardy,

    Please keep in mind that the datasheet (page 81) specifies a maximum load capacitance of 100 pF, which is x1000 less than 100 nF. With such a large capacitor connected to the output, this high current behavior would make sense fundamentally, since the capacitor is trying to charge up (remember that a discharged capacitor is like a short circuit). The datasheet also discusses load resistance on page 81, which is important.

    Since the DAC12 is an output here, it sounds like you are trying to mitigate EMC on this signal going to another component, so I'd recommend finding out what EMC recommendations are given for that specific component's inputs. I'd suggest taking a look at the following resources, which are focused on PCB design strategies for reducing EMC/EMI.

    Regards,

    James

    MSP Customer Applications

  • 0 in reply to James Evans
    Expert 2870 points
    Hey, many thanks James. The maximum load capacitance explains the behaviour. Actually we have removed the C connected to the DAC output and now have just one RC circuit with 9k4Ohm followed by 1uF.
  • 0
    Guru 74080 points

    Hardy Griech said:
    So we both are wondering why this could happen, because our idea of the MSP430 internals is, that there is a simple amplifier at the output which drives the analog signal.

    Oscillation with a high current draw is a common problem when connecting a load capacitance to an amplifier. You will rarely see a capacitor directly connected to an OPAMP output. A low value series resistor normally helps to prevent oscillation.

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