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ADS8688: Technical questions

Part Number: ADS8688
Other Parts Discussed in Thread: ADS8684

Hello, 

I am currently planing to add the ADS8688 to one of my designs. After having read the datasheet, I still have a few questions that I ask here. 

1) Concerning the analog input channels, what can I do about the unused ones ? Can I left them floating ? 

2) In the datasheet, I sheet that an additional low-pass filter is advised to filter out noise from the analog inputs of the device. Which cutoff frequency and components values do you advise for such a filter ?

Thank you for your answer.

  • Hello Antoine,

    Thanks for considering ADS8688 in your design.

    1. The unused channels can be shorted to the analog ground. If you only need 4 input channels, we do have a 4-channel version, ADS8684 which is fully compatible with ADS8688 except channel count.

    2. ADS8688 has an integrated analog front-end (AFE) on each channel including a low-pass filter(LPF), the BW of LPF is 15kHz(-3dB).  If a lower BW is desired regarding your input signal and application requirement, an external passive LPF should be designed and used. After you confirm the BW of LPF, the resistor and capacitor value can be determined(1/2piRC), notice that the input impedance of ADS8688 is 1Mohm, hence a large series resistor of LPF can lead to an additional gain error because of resistor divider from the series resistor and input impedance of ADC.

    Regards,

    Dale