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ADS1235EVM: Antialiasing filter selection

NIMIT VACHHANI
Prodigy 190 points
Part Number: ADS1235EVM
Other Parts Discussed in Thread: ADS1235, ADS1258

Greetings Everybody,

I am using ADS1235 evm. I want to filter analog signals and then feed to ADC before sampling. ADC1235 evm is running fine, all i want is more stable readings. Can anybody guide me how to select antialiasing filter for my requirement.

Thanks.

  • 0
    TI__Mastermind 44510 points

    Hi NIMIT,

    While it is possible that aliasing could be resulting in unstable readings, it is probably more likely that other noise sources are to blame for this kind of behavior...Often times noisy reference voltages or poor layouts can result in poor ADC noise performance. In some cases, there may be excessive ringing on the power supply lines due to LC resonance, or perhaps the reference source is oscillating because it has not be compensated properly.

    It would be a big help if you could share a schematic of your circuit!

    Regarding anti-aliasing filters, I recently answered a similar question for a different delta-sigma ADC, the ADS1258, but my recommendations would still be the same for the ADS1235... 

    Passive input RC filters for low-bandwidth delta-sigma ADCs are generally used for anti-aliasing. The SINC digital filter of the ADS1258 makes for a good low-pass filter (LPF) with a stepper roll-off than a single-pole filter. Below is an example of the ADS1258 SINC5 filter (with 64 averages) compared to an RC filter with a cutoff of about 3.3kHz. Notice that the SINC filter passband repeats at multiples of the modulator sampling frequency so the RC filter attenuation around 8 MHz helps prevent aliasing, while the SINC digital filter does most of the noise filtering.

    You can further reduce the cutoff frequency of the RC filter to provide additional attenuation in the very low-frequency range…

    …but I would be careful when doing so and would recommend the following limitations:

    • Limit the series resistor value to no more than a few kOhms!
      Above ~3kOhms, the resistor’s thermal noise will start to exceed the noise floor of the ADC and will degrade the noise performance of the overall system. Large register values can also present a problem when ADC inputs are unbuffered, so do use an input buffer between the MUXOUT and ADCIN pins and make sure any large RC filters are placed in-front of this buffer.

    • Limit the shunt capacitor value to allow the use of a high-quality C0G-type capacitor!
      To achieve higher capacitance values, you either have to use a larger surface mount package or use a different type of dielectric material. C0G ceramic capacitors are some the most stable dielectrics which will not change as much over temperature, applied voltage, and offer lower leakage currents than other types of capacitors.

      Last I checked, C0G type capacitors can provide the following capacitances for a given SMD package size:
      • 0201 package: up to 0.1-10nF
      • 0402 package: up to 2.2nF
      • 0603 package: up to 15nF
      • 0805 package: up to 47nF
      • 1206 package: up to 120nF

     

    Also keep in mind that RC filters with lower cutoff frequencies take longer to settle. Loading effects from multiple RC circuits in series combined with long settling times can lead to unstable ADC readings. Therefore, if a higher-order LPF is needed, consider using an active filter in place of multiple passive filters.
  • 0 in reply to Christopher Hall
    Prodigy 190 points

    Thanks Chris,

    I will try it and see what happens. I might take few days to try this out as i am stuck with some other work.

    Thanks again.