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ADC128S102 Va capacitor recommendation

and
Prodigy 205 points
Other Parts Discussed in Thread: ADC128S102, LM4120

Looking for some clarification on driving the Va pin (analog) of the ADC128S102 using the LM4120.

The EVM for the ADC, document snau167 shows 1000pF + 0.047uF on the Va pin of the ADC.

(a)

Document snvs049c for the reference, the LM4120, p.11, "Application Hints" states that values above 0.047uF must be tantalum. What are the recommended capacitance values (types, and ESR) to use when using the LM4120 to power Va? Document snau167 shows Va being driven by the LM4120 - is this design recommended?

(b)

Looking at figures 24 or 27 of snvs049c, can a transistor be added to allow increasing the load capacitance of the LM4120?

  • 0
    TI__Guru 71526 points

    The recommendations in the LM4120 datasheet are for the COUT at the chip.  In the EVM design, the added 0.001uF cap is at the ADC, so the effect at the LM4120 is separated by the trace and jumper inductance.  In addition, the tantalum reference in the LM4120 datasheet is in reference to 1uF and above capacitors (  With tantalum capacitors, in the 1μF range  ) and the aforemention 1000pF cap is well below this value.  Bottom line, no need to be concerned with the additional cap at the ADC.

    As far as the type and ESR, again, in that same datasheet paragraph, the MIN 0.022uF to 0.047uF caps are stated as ceramic, having an ESR: 

    ESR of about 0.1 to 0.5Ω. Smaller ESR can be tolerated, however larger ESR cannot. 

    ~Leonard