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MSP430F425A: SD16, Delta-Sigma Convertor

Nick Price
Expert 2265 points

Part Number: MSP430F425A
Other Parts Discussed in Thread: ADS1120

I am struggling to find information of the conversion rates available for the MSP430F425A built in triple delta-sigma.

I need to sample over a 20mS period, I am running a battery operated product. So I can't sample for to long.

Regards

Nick

  • 0
    Guru 46710 points

    Nick Price said:
    I need to sample over a 20mS period, I am running a battery operated product. So I can't sample for to long.

    You definitely shall not try to use 20ms ADC sampling time. It is not possible to run SD ADC so slow anyway. Instead just add RC low pass filter between signal source and ADC input, average signal using capacitor, not ADC.

  • 0 in reply to Ilmars
    TI__Guru 56685 points
    Hello Nick,

    This type of ADC works fundamentally different from the typical SAR based ADCs found on MSP430s. That being said, your sampling frequency is going to be in relation to your over sampling ratio (OSR) and your modulation frequency. Your modulation frequency needs to be chosen in relation to the external circuit's settling time as well. Please see sections 29.2.4 (Analog Input Setup) - 29.8.7 Digital Filter in the device user guide for more information about all of these concepts.
  • 0 in reply to Ilmars
    Expert 2265 points

    Normally I use a ADS1120 which has an internal 50Hz filter.   But this product is more cost critical. The 20mS sample is to try and cancel out the maximum amount to 50Hz mains noise.

  • 0 in reply to Nick Price
    Guru 46710 points

    Nick Price said:
    Normally I use a ADS1120 which has an internal 50Hz filter.   But this product is more cost critical. The 20mS sample is to try and cancel out the maximum amount to 50Hz mains noise.

    Oh 50Hz rejection :) You shall read chapter 30.2.7 Digital Filter then. It says:  First notch of sinc3 comb filter is equal to F_notch = F_modulation/OSR. The notch’s frequency can be adjusted by changing the modulator’s frequency. For example at OSR=1024 notch will hit 50Hz if ADC clock frequency is 50*1024 = 51.2 KHz. Sorry, most likely I was incorrect initially by telling that modulator frequency will be too low for 20ms output period. 51.2KHz modulator freq could be fine thou I have no experience running SD16 so "slow", datasheet tells only max figure. Note that SD16 have low power mode, SD16LP = 1.

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