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ADS1271 frequency transfer function

Harald Hellmann
Prodigy 125 points
Other Parts Discussed in Thread: ADS1271, ADS1298, ADS1274, ADS1278

Hello,

I'm looking for the frequency transfer function of the ADS1271 in a way like it is given for ADS1298 as equation 5. Does anybody know where I can find it?

Thank you in advance.

Harry

  • 0
    TI__Mastermind 43915 points

    Hi Harry, 

    The transfer function for the ADS1278 is not so cut and clear. First of all, the ADS1298 and the ADS127x series parts use completely different filter implementations. The ADS1298 uses a Sinc filter while the ADS1274 uses a FIR filter. Converters with a FIR filter implementation have the benefit of the steep roll off around the Nyquist frequency but require the tradeoff of requiring multiple conversion results to do so. In this case, you can see that the ADS127x group delay for a step signal is 36/38 conversions meaning there will be a fail amount of delay from when the input signal is applied at the input to when the output changes. Below are a few excepts from other posts explaining the FIR filter in the ADS127x.

    The architecture of the ADS1274/8 delta sigma converter uses a FIR filter to average multiple readings. In this specific ADC, you can think of each digitized output result as an average of 78 conversions. As another conversion is is taken, it is added to the previous 77 conversions and averaged using weighted coefficients. So, from a DC input step, the converter will need 78 full conversions to clean out the filter with data corresponding to the new input signal. With each /DRDY pulse, new samples are converted and averaged to the output data. Unfortunately, outside of what is written on page 22 of the ADS1274/8 data sheet, there is not too much more we can share.

     

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 125 points

    Hello Tony,

    thank you for your information. I thought about a FIR filter within the ADS1271 and I understand if these coefficients are a TI secret and should not be published.

    But  maybe there is a way to do an algebraic functional representation of the transfer function without the internal coefficients like you sometimes do with Pspice model. These model behave at the inputs and outputs like the chip they simulate without the whole internal circuits. Because I should do a documentation about a product of us where we use the ADS1271 and I want to draw the transfer function like you do in Fig. 54 to Fig 57 of the data sheet.

    Thank you in advance for any further information.

    With Kind Regards

    Harry

  • 0 in reply to Harald Hellmann
    TI__Mastermind 43915 points

    Hi Harry,

    The FIR filter coefficients are TI internal information. We have an excel tool for a few of Delta Sigma ADCs that include FIR filters to help show frequency response of the filter. Unfortunately, we do have anything like this for the ADS127x family of parts. I wish I could be more help. If you have any specific questions about the filter, please ask and I will do my best to answer.

    Regards,

    Tony Calabria