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ADS1232: What is FD ?

user5093154
Prodigy 80 points
Part Number: ADS1232
Other Parts Discussed in Thread: ADS1220

Hello,

Guys i have some questions. What is FD ? So Output data rate ? I want to do anti aliasing filter. But i need filter formula. Thanks.

  • 0
    TI__Guru** 113765 points
    Hi user5093154,

    I think maybe you are asking about cutoff frequency for the input filter, Fc. As the ADS1232 is an oversampling ADC, you don't need to have a hard filter cutoff and can use a simple RC filter at the input. You could be very strict with making a cutoff frequency relative to output data rate, but this produces a very long analog settling time for the filter. You should use the modulator frequency.

    The actual sampling rate of the modulator is 76.8kHz. As discussed in the datasheet there already is some filtering to remove input chopper residue when using the higher gains. See the the Bypass Capacitor section on page 14 of the ADS1232 datasheet.

    So there is no specific formula to determine where you want your filter to start cutting off. Where you decide to set your filter values is determined by what you want to filter and if you are using lower gain or higher gain. At low gain you want to keep your resistor values small to prevent error from the switching bias currents through the inputs.

    Generally, good starting values are from 1k to 4.7k for the resistors, and 10nF to 100nF for the differential cap between inputs. You do not need common-mode caps, but if desired make sure that the values used are 1/10 or less of the values used for the difference cap otherwise you can see issues with the filter having a mismatch between inputs that cause a difference voltage.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Prodigy 80 points

    I understand that, but Bonnie Baker gave it a filter calculation. Essentially I want to delay the settling time. So the input frequency will be half the sampling frequency. How can I do that ? Could the filter you mention be a standard account?

    Thanks for your reply

  • 0 in reply to user5093154
    TI__Guru** 113765 points
    Hi user5093154,

    I'm not sure what you mean by 'delay the settling time'. The ADS1232 is a device designed to measure DC or very slow moving signals. The sensor signals, such as a thermocouple or load cells, are of no significance themselves to the effects of aliasing. But excitation sources, or EMI/RFI can induce higher frequency noise that should be filtered otherwise those frequencies can fold back into the pass band of the ADC. The concern with the ADS1232 is to prevent aliasing of the higher frequency content into the pass band of the the ADC.

    The modulator sampling frequency of the ADS1232 when using the internal oscillator is 76.8kHz. I/2 of 76.8 KHz, is 38.4kHz. However, due to the low-pass digital filter in the ADS1232, there is no reason to design the filter for a sharp roll off at that high of frequency. Instead you can design using a 1st order RC filter with 20dB/decade rolloff (where Fc = 1/(2piRC)).

    There are several approaches to the filter values chosen. One is to design a reasonable RC filter where the Fc (or 3dB cut off frequency) at a point where Fc is 40 to 60dB from 1/2 of the sampling frequency. As an example you could decide to use 384Hz as Fc. This will provide a filter where at 1/2 the sampling frequency the content will be 40dB down. This may not seem to be a good filter response with only 40dB, but remember that the noise may be 60-80dB down already from the signal of interest and the combined response will be 100-120dB down from the 1/2 sampling rate.

    Another approach would be to create a filter based on the output data rate which can be done. However, the component values for an RC filter now start to become quite large. An active filter could be used, but now additional noise and error will be added to the measurement.

    A third approach is to design around a known frequency that you want to filter, such as power line-cycle pickup. However, there is filtering by the digital filter when using 10sps. At 80sps, the line-cycle noise is within the pass band of the ADC, so here there may be a desire to set the Fc lower to cover these frequencies.

    In any design, it is best to prevent any external noise sources from getting into the analog inputs in the first place. This may require ferrites, TVS diodes and shielded cabling that is properly terminated.

    If you tell me what sensor you are wanting to use and the environment the measurement will be taken I may be able to help with the design. If I'm not understanding what you are asking for please explain in more detail what you are specifically asking for. If you want further information regarding input filtering take a look at the ADS1220 datasheet in section 9.1.2 on page 45 where there is a specific topic on Analog Input Filtering. Even though the ADS1232 differs quite a bit from the ADS1220, the input filtering discussion still applies.

    Best regards,
    Bob B