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ADS1291: ADS1291 VCAP1, VCAP2 Values

Karthik P R
Prodigy 60 points
Part Number: ADS1291

Tool/software:

Hi Team,

In the datasheet for ADS1291, it is recommended to use a tantalum capacitor for VCAP1, but the specific values for this capacitor are not provided. Would you be able to recommend appropriate values? Currently, we have used a 1µF tantalum capacitor for both VCAP1 and VCAP2. We noticed a slight difference in noise levels after the change; previously, we had ceramic capacitors on VCAP1 and VCAP2, and switching to the 1µF tantalum capacitor resulted in lower noise.

Thanks and Regards
Karthik P R

  • 0
    TI__Guru 66446 points

    Hello Karthik,

    Thank you for your post. 1µF is the correct value for VCAP1 and VCAP2. Figure 60 shows the recommended bypass capacitor values:

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 60 points

    Hi Ryan,

    Thank you for your response. Since we are using internal references, would it be better to use tantalum capacitors? We intend to use C0G capacitors for VCAP1 and VCAP2, but 1uF C0G capacitors are unavailable. If we go with tantalum capacitors, what should be the equivalent series resistance (ESR) of the capacitors? The datasheet does not specify this. We aim to achieve the best possible performance from the chip. Additionally, are VCAP1 and VCAP2 the analog bypass capacitors, and do they function as the output capacitors for the internal LDO? Could you please help us understand which option would provide the best noise performance?

    Thanks, and Regards
    Karthik P R

  • 0 in reply to Karthik P R
    Prodigy 60 points

    Hi Team,

    Could you please respond to the above thread.

    Thanks, and Regards
    Karthik P R

  • 0 in reply to Karthik P R
    TI__Guru 66446 points

    Hi Karthik,

    Either Tantalum or C0G capacitors are fine to use. There is no requirement for minimum ESR as this node follows the internal R1 resistor (100kΩ or 84kΩ) and feeds into the high-impedance input of the reference buffer. Please refer to Figure 30. 

    Both VCAP1 and VCAP2 are used to store charge and filter internal analog voltage nodes. There is no LDO in the ADS1291.

    Regards,

    Ryan