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ADS127L01EVM: Gain/LF

Chiaki Tanaka
Prodigy 40 points

Part Number: ADS127L01EVM
Other Parts Discussed in Thread: THS4551, ADS127L01, ADS127L11

About gain,I would like to amplify the signal input at AINN/AINP by 100 times.
I believe I can achieve this by changing the resistor values around U30.
Is this correct?
If so, could you please tell me how to set the resistor values appropriately? 

Also about low-pass filter,I would like to implement a low-pass filter with a cutoff frequency of 5kHz.
I believe I can do this by changing C90, R77, and R82.
Is this correct?
If so, could you please tell me how to set the component values to achieve a 5kHz cutoff?

  • 0
    TI__Guru 51280 points

    Hello Chiaki,

    Yes, the RC values can be updated for a gain of 100 and 5kHz f-3dB low-pass frequency.

    The output filter should not be changed; these values are optimized to drive the ADS127L01 input using the THS4551.  However, the other component values can be updated/installed to implement a 2nd order low pass filter response.

    1. Change R74, R79 from 1k to 95.3 (G=100)
    2. Change R71, R84 from 1k to 9.53k (G=100)
    3. Change R75,R89 from 0 to 107 (5kHz filter)
    4. Install C88 with 220nF C0G/NP0 (5kHz filter)
    5. Install C89 with 22nF C0G/NP0 (used for amplifier stability, not part of filter response)
    6. Install C86, C92 with 2.2nF C0G/NP0 (5kHz filter)

    Also attached is the TINA-TI SPICE file:

    THS4551_100G_5kHz_Transient.TSC

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 40 points

    Hello, Keith.

    Regarding items 4 and 5, I can't find capacitors with capacitances of 220nF and 22nF that have a C0G temperature characteristic and come in the 0603 size.

    Would X7R temperature characteristics be acceptable instead? Or would it be better to change other conditions rather than the temperature characteristic?

    Also, if there are capacitors that meet all the conditions, please let me know.

    Thank you,

    Chiaki

  • 0 in reply to Chiaki Tanaka
    TI__Guru 51280 points

    Hello Chiaki,

    If you want to maintain low distortion (good THD and linearity), then you need to use these types of capacitors.  X7R are fine for DC signals, such as reference voltage filter and power supply bypass capacitors, but they will add distortion to any AC input signal.  Film capacitors can also be used, but they are much larger in size than ceramic capacitors.

    Based on the existing design with gain=100, you will have input noise (referred to the amplifier input side, before gain) of about 0.3uVrms.  If you reduce the gain of the amplifier to gain=10, the input noise will be around 1uVrms.  Using a lower gain will also reduce the size of the capacitors needed, with a maximum value of 18nF, which is available in an 0603 package size.  If you can tolerate the higher measurement noise, this is an option. 

    Another option is to use the latest generation ADC, the ADS127L11.  This ADC has more digital filter options, which will allow setting the cutoff frequency to 5kHz.  In this case, your input filter only needs to act as an anti-alias filter, and the cutoff frequency can be set to 50kHz, which will also reduce the filter capacitors to a maximum value of 18nF.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 40 points

    Thank you Keith,


    I will consider the suggestions you provided.


    Additionally, I have one more question.
    When configuring a digital filter (using the ADS127L11 or a microcontroller, for example), I believe only resistors for setting the gain are needed, and capacitors do not need to be implemented. Is that correct?
    According to the response I saw, it says that if a digital filter is configured with the ADS127L11, the filter capacitors can be made smaller. However, I’m wondering—if a digital filter can be configured, wouldn’t that mean filter capacitors are not needed at all?

    Thank you,
    Chiaki

  • +1 in reply to Chiaki Tanaka
    TI__Guru 51280 points

    Hello Chiaki,

    When using a digital filter, you still need an input filter to attenuate any input frequency that is greater than the modulator frequency (input sampling rate), otherwise, you will see an aliased version of the signal.  Depending on the over-sampling ratio (OSR), which is the ratio of the input sampling rate (fMOD) to the output data rate (fDATA) of the ADC, you may only need a simple RC filter.

    Take a look at this TI precision labs that discusses this topic.

    https://www.ti.com/video/6242062186001

    Note: If you have a bandwidth limited system, where the maximum input signal frequency is known, then you may not need any additional filtering.  An example case would be low frequency accelerometers that may only have a frequency bandwidth of 50kHz.  As long as the maximum signal frequency is less than the input sampling frequency (to be precise, fMOD-fDATA/2), then you typically only need a simple RC filter to filter external noise that may couple into the system.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Prodigy 40 points

    Hello, Keith.

    Thank you for your response.
    Based on your response, I’m planning to implement a 50 kHz anti-aliasing filter before the ADC and a digital low-pass filter with a 5 kHz cutoff after the ADC.

    Thank you,
    Chiaki