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MSP430F2618 strange problem with analog outputs

Massimiliano Sanna
Prodigy 110 points

Other Parts Discussed in Thread: MSP430F2618


Hello I'm planning to use the two analog outputs of the MSP430F2618. The final circuit will have opamp buffers to allow more output current but I'm observing a strange behavior when setting the outputs (with no load): if I set the first analog output to give, for example, 1V, I get the desired voltage value. Then if I set the second analog output to get whatever value of voltage (from 0V to 2.5V) I observe that the first output drops to 0.994V. Setting the second output cause a voltage drops of 6mV and for my application is intolerable. The code to set the first output is:

DAC12_0CTL = DAC12SREF1 + DAC12IR + DAC12AMP1 + DAC12ENC;
delay(20); // delay of 20ms
DAC12_0DAT = code;

The code to set the second output is identical but with DAC12_1CTL and DAC12_1DAT. I'm using a 2.5V external reference connected to Veref+ (pin 10) and outputs on P6.6 and P6.7. The MSP430F2618 is put in LPM3 but I observe the same problem also in active mode and with different values of DAC12AMP. Can anyone explain what is happening? So we can find a solution. Thanks a lot

Massimiliano

  • 0
    Guru 227245 points

    Activating the second DAC may increase the load on the reference and cause a reference voltage drop. The additional output current may cause a voltage drop on AGND, affecting the reference too. That’s what might happen even if the MSP is working as expected.

    Check where you put your GND reference point when measuring the output signal. (best would be the AVSS pin of the MSP)

  • 0 in reply to Jens-Michael Gross
    Prodigy 110 points

    Thank you for the reply. I'ts a very good point and I will try to increase the polarization of the reference voltage. The output current are not a problem because I put buffers in front of the outputs and regarding the GND, It's connected to AVSS. So It may be, like you say, the increased load on the reference. I'll check this soon. Thanks a lot!!!!

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