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ADS1261: Chopper stabilization frequency

Carlo
Expert 7715 points
Part Number: ADS1261

Hi team,

Good day.

In the datasheet, it was stated that "The PGA and modulator are chopper-stabilized at high frequency in order to reduce offset voltage, offset voltage drift and 1/f noise". What is the chopper stabilization frequency?  It is the same as the fCLK Frequency which is typically 7.3728 MHz?

Regards,
Carlo

  • 0
    TI__Mastermind 45715 points

    Hi Carlo,

    I will check and get it back to you soon, thanks.

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Expert 7715 points

    Hi Dale,

    Thank you for looking into this.

    Also, can you confirm the value of multiplexer resistance RMUX? The datasheets states that "When using the burn-out current sources, be aware of the offset error caused by the currents flowing through impedances in the input path, including the multiplexer resistance RMUX".

    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Mastermind 45715 points

    Hi Carlo,

    The chopper stabilization frequency is fmod/OSR and the lowest frequency is fmod/64. Also, fmod=fclk/8, so 

    fchop=fclk/(OSR*8).

    fmod is modulator clock.

    fclk is master clock which can be found from following table.

    For your second question, the internal multiplexer resistance RMUX should be very small, I will check with team and get it back to you.

    Best regards,

    Dale

  • 0 in reply to Dale Li
    TI__Expert 7715 points

    Hi Dale,

    Thank you for sharing the equation for achieving the chopper stabilization frequency.

    I'll be waiting for your update regarding the multiplexer resistance RMUX.

    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Mastermind 45715 points

    Hi Carlo,

    The multiplexer resistance RMUX is between 200 to 500 ohms from design perspective as a fair estimation, it depends on the absolute voltage of the input pin.

    Thanks&Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Expert 7715 points

    Hi Dale,

    Good day.

    Can you please explain why the  RMUX is dependent on the absolute voltage of the input pin? According to our customer, resistance is an intrinsic property of a material, not dependent on the magnitude of a voltage running across it. If a high voltage is applied, the generated current sufficiently heats the resistor, which changes the resistance, but that wouldn't be expected for the range of inputs expected on the mux pins.


    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Mastermind 45715 points

    Hi Carlo,

    The on-resistance(Rmux) of multiplexer is not constant, it is a function of supply voltage / input voltage since it’s a regular Tgate structure (PMOS and NMOS in parallel turning on through 0V/supply voltage). PMOS conducts for positive input voltage and NMOS conducts for negative input voltage. Rmux is the combination result of both N channel and P channel FETs. Please see following equivalent circuit and corresponding curve.


    For detailed information, please check TI Precision Labs - Switches and Muxes: On-Resistance Flatness and On-Capacitance.

    The technical documentation is available as well: Basics of Analog Multiplexers 1.

     

    Regards,

    Dale