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REF6025: output capacitor

Part Number: REF6025
Other Parts Discussed in Thread: ADS8887, , ADS8881EVMV2-PDK, ADS8881, REF6045

Tool/software:

I am using the REF6025 and ADS8887 in a design, with the REF6024.

The datasheet for the REF60xx series, on page 23, in section 9.3.4 Stabilitypage 23,  states that the output capacitor needs to be from 10uF to 47uF with ESR in the range of 20mohms or 5 mohms to 100 mohms.  (See the texet and figure below).

The ADS8881EVMV2-PDK evaluation board, uses the ADS8881 and REF6045.   The user's guide for the EVM, in the BOM on page 19, lists for this output capacitor a Murato GRM32ER71A476KE15L   The datasheet for that capacitor reports its dissipation factor as DF </= 0.1.  Using esr = DF/(2 pi f C),  we find that that capacitor would seem to have an ESR </ = 2.8 ohms.

So the output capacitor in the EVM seems to not be compliant with the stability requirement stated in the REF60xx datasheet.

1)  Is one of those documents in error?  (which one?)

2) Is the upper bound on ESR actually higher than 100 mohm?

3) What is the ESR requirement based on?   Is there an equation?


For now I am designing with the Murato part specified in EVM BOM,    Please advise if I should change this.

Thank you

The REF60xx family of voltage references are stable with output capacitor values ranging from 10 µF to 47 µFAt a low output-capacitor value of 10 µF, an effective series resistance (ESR) of 20 mΩ to 100 mΩ is required for
stability; whereas, at a higher value of 47 µF, an ESR of 5 mΩ to 100 mΩ is required. The shaded region in
Figure 53 shows the stable region of operation for the REF60xx devices.

  • Hi Mitch,

    Thanks for reaching out.

    I took a look at the GRM32ER71A476KE15L information, and from what I can find from Murata's website, the ESR is below 100mΩ from about 1kHz to about 1GHz. Given the bandwidth of REF6025, GRM32ER71A476KE15L should work fine for the output capacitor. I would suggest to follow the suggestion in the datasheet of adding a footprint for an additional ESR with the capacitor, like so:

    In this case, I would not be concerned about this capacitor, as it can be used and still fit the datasheet requirements. The ESR requirement is based on the stability conditions of REF6025, there is no equation other than Figure 53. 

    Thanks,

    Jackson