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ADS1278-SP: Input capacitance low frequency application

Jonas Felder
Prodigy 15 points
Part Number: ADS1278-SP

Tool/software:

Dear TI,

We have an application with a low frequency differential input signal (typical 200mHz, max. 5Hz).
The Datasheet tells that a 1nF to 10nF capacitor should be used directly across the analog inputs. 

Since we have low frequency signals the internal digital filter has a too high cutoff frequency.
Can we use bigger capacitors across the input (200-350nF) for our anti-aliasing filter? 
Or must the recommendation of max 10nF be adhered to due to the internal circuit design?

Regards

Jonas

  • +1
    TI__Mastermind 46550 points

    Hello Jonas,

    The 10nF recommended max is limited by the ability of an input amplifier to drive larger capacitance values and still maintain signal bandwidths in the 10kHz to 50kHz range.  However, since your maximum signal bandwidth is 5Hz, you can make this input capacitor as large as needed as long as your input amplifier can drive this value and maintain good phase margin.  Also, you will want to use an NP0/C0G or equivalent type of capacitor to maintain low distortion.  These types of capacitors are usually limited to 100nF, although there may be slightly higher value options available.

    Also, you can lower the clock frequency to a minimum of 100kHz.  Using the high-resolution mode, as an example, will result in an output data rate of 195sps, and a -3dB bandwidth for the digital filter of 95.7Hz.

    Regards,
    Keith Nicholas
    Precision ADC Applications