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Original question:

ADS124S08:

ADS124S08: REFOUT capacitor can affect the output code

Lin_Charles
Expert 4260 points

Part Number: ADS124S08

Hi team,

   We know that the internal reference requires a minimum 1-µF capacitor connected from REFOUT to REFCOM and should be placed as close as possible. I find one thing that,

If this capacitor is far away from REFOUT pin, the output code will be higher than the actual value and have a positive fixed deviation. Could you please help explain this phenomenon? If any document can be shared will be much useful. Thanks.

  • 0
    TI__Guru** 113765 points
    Hi Charles,

    I do not know of any specific document or application note regarding the internal reference and cap placement. The Figure 121 in Section 12.2 of the datasheet shows an example layout.

    The capacitor on REFOUT is for stability as well as noise reduction. For stability, a minimum of 1uF is required. If you monitor the REFOUT pin with an oscilloscope you may see an oscillation. By moving the capacitor further away from the device pin you will be adding resistance and inductance which will affect the stability of the reference.

    It is also important that REFCOM be properly connected to AVSS (sometimes this is missed). If the system is using a unipolar analog supply (+5V for example), then the REFCOM should be connected to AGND. You also need to make sure that the capacitor also has a low impedance connection to AGND to keep the overall inductance low.

    Another issue that can cause an error with the internal reference is by turning on the REFN_BUF reference buffer. The buffers cannot drive all the way to the supply rails, so when using the internal reference the REFN_BUF should be disabled.

    Best regards,
    Bob B