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SD16_A noise from adc inputs when conversion is started

Chad Williams90069
Prodigy 30 points
Other Parts Discussed in Thread: MSP430F4793

I am using MSP430F4793 analog input 0

I find that when conversion is started some noise is generated. I assume this is because the SD16 is being powered up whenever a conversion is started in single coversion mode.

8765.scope_0.tif

I am running a single conversion each 250mS.

When I run the ADC in continuous mode, I don't see this noise. However its a battery powered application and  I can't run the ADC in Continuous mode.

The ADC input is being driven by an OpAmp as a preamplifier. I I install 10K resistors between the output of the opamp and the ADC input the startup noise is reduced by a quarter, but this is not the best solution and doesn't eliminate the noise. I have tried using the internal reference instead of an external reference, but to improvement occured.

It appears as a decaying oscillation which may be a clue.

I have tried many things in software and hardware, but cannot eliminate the noise.

  • 0
    Prodigy 30 points

    I have since been able to rectify the issue. the A0.0-  pin was connected to a 0.9V reference which had a 470nF cap in place for the goal of reducing noise & RFI susceptability. Actually the 470nF cap was not necessary so I removed it and this eliminated the oscillation that occurred when the ADC was started. Problem solved.

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