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ADS1209: Specified Differential input voltage range

Part Number: ADS1209
Other Parts Discussed in Thread: TMS320F28377D

Hello Team,

I have a customer who have questions with respect to the ADS1209 :

For example If we apply 1VDC between (CHA+ and CHA-) , we expect to read a raw value of (1V * (2^16)/5V) = 13107 counts through the TMS320F28377DPTPS.

 But the data that we actually read is 12058 counts.

Page 3 of the datasheet under FSR, the Specified Differential input voltage range is Vref * 0.92. With this calculation the count of 12058 seems correct. Can someone explain me what this spec actually means and how the value of 0.92*Vref was determined. 

From what I understand while reviewing the Sigma delta converters for MSP430 in previous cases is it should be something like this :

Input voltage range of 100% of the FSR, 1.00* |Ch+ - Ch-| <= 1.00*Vref = 1.151 V
Input voltage range of   80% of the FSR, 0.80* |Ch+ - Ch-| <= 0.80*Vref = 0.80*1.151 V = 0.921 V 

Beyond these 80%, the specified performance isn't guaranteed. So if you exceed them, your results are less reliable.

Is the understanding correct?

Thank you for your time.

Kishen

  • Hi Kishen,

    The SD in the MSP430 is a little different - it's got a fixed filter configuration. For the ADS1209, you are taking a bit stream and then filtering that with the TMS320F28377D. The FSR of the ADS1209 is the point at which the analog input provides 100% '0's' or '1's' in the digital bitstream output, resulting in filter saturation. The linear range of the modulator is then set at some point (in this case, 92%) below the saturation level in order to keep the filter from saturating.
  • Hello Tom,

    Thank you for the clarification. I appreciate your help.

    Kishen
  • So If I understand correctly, the ADS1209 apply a gain of 0.92 in the “2nd-order ΔΣ modulator”, is that correct ?
  • Hi Kishen,

    No, there shouldn't be any attenuation from the modulator, but there may be something in the filter setup of the TMS320F28377. Can you tell us how you have the filter set up in your controller?
  • Hello Tom,

    Thank you for the continued support.

    The Filter is set up as follows :

    " The SDM is setup consistent with the example code from TI. We are configured for “Mode0” (mod clock = mod data rate), mod clock is set from the ADS1209 (20Mhz/2 = 10 Mhz). We are configured for a SINC3 filter, an OSR of 250 (40 kHz update), and a data shift of 9 bits (per Table 13-5 in the manual).

    To initialize we use the function Sdfm_configureData_filter from F2837xD_sdfm_drivers.c (TI example code). This is what the function call looks like for each filter (I’ve chose FILTER1 here):

    Sdfm_configureData_filter(PeripheralNumber, FILTER1, FILTER_ENABLE, SINC3, OSR_250, DATA_16_BIT, SHIFT_9_BITS); "

    Regards,

    Kishen

  • Hello Tom,

    Is there an update to this issue. The customer wants to know where the error or scale factor comes from. Do I have to look at the F28337D ?

    Thank you for your time.

    Kishen
  • Hi Kishen,

    Sorry for the delay.  There may be something in the code, perhaps an offset or something.  Is it possible to input a sine wave, or triangle wave at +/-2.5 and get a data dump of the conversion results?

  • Hello Tom,

    Here below are screenshots showing an input signal of +/-1.36V @ 10Hz at the SDM input.

    The customers design and test bench setting do not  allow them to apply more than +/-1.36V in AC mode.

    In nominal mode they will never go higher than 1.36V at the SDM input.

    Yellow trace (V_ab) is the measure with a regular ADC and it match perfectly with what is expected. ( It’s the ADC that is integrated in the microcontroller TMS320F28377DPTPS )

    Purple trace (V_absdm) is the measure provide by the SDM ADS1209.

    Scale factor apply on both channel : 4200 = 1.36V

    below is a screen shot with step response.

    Would you know what is causing the SDM to read like this ?

    Again, Thank you for your attention towards this post.

    Sincerely,

    Kishen

  • Hi Kishen,

    Have you made any headway with this issue?
  • The customer was able to resolve the issue. He changed some of his code settings in particular the OSR to 256 from OSR = 250 as per you suggestion.

    Thank you very much for the support and suggestion and apologize for the wait in updating this post.

    Sincerely,

    Kishen