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Original question:

ADS8508: Selecting Capacitors for CAP, REF, VANA

ADS8504: Question regarding ESR for external caps

Darren (FAE)
Expert 5483 points
Part Number: ADS8504
Other Parts Discussed in Thread: ADS8509, ADS8505, ADS8508, , ADS7804

In another post we discussed some details regarding the ESR requirements.
https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/1131257/ads8508-selecting-capacitors-for-cap-ref-vana/4360073?tisearch=e2e-sitesearch&keymatch=darren%20ads8508#4360073 

To recap, the end result was that:

- ADS8509 DS Figure 23 shows ESR vs SNR. We can see that the device is designed for lower ESR caps.
- ADS8505 DS Figure 17 shows ESR vs SNR. We can see that the device is designed for a wide range of low-high ESR caps.
- The summary was ADS8508 has a similar structure to ADS8509 and therefore works better for lower ESR caps than higher ESR caps. But there was no similar ESR vs SNR graph.

The question I have now is regarding ADS8504.
Is this the same type of device as ADS8505, just with a parallel output I/F vs the serial I/F the ADS8508 has?
If so, then should the ESR graph of the ADS8505 be similar to Figure 17 of ADS8505 DS, or similar to Figure 23 of the ADS8509, or is it a different design and has it's own characteristics...?

This is needed to help select the right capacitors.
There is some similar output code variation observed in the current design; similar to what was seen in the ADS8508 (solved by switching from Tantalum to Ceramic)

EDIT:
Same question for the ADS7804. What does the ESR / SNR plot for the device look like?
Is there internal compensation for wider ESR range, or not? etc...

Regards,
Darren

  • +1
    TI__Guru**** 171726 points

    Hi Darren,

    Yes, the ADS8504 is based on the same process as the ADS8505 only the interface has changed.  The ESR on the CAP pin for the ADS85xx devices, should ideally be kept under 3 ohms.  The ADS7804 and the rest of that family, are built on a different process.  The ADS78xx devices came from the original Burr-Brown corporation in the early/mid 1990's before quality ceramic capacitors were widely available which is why that series of devices mentions tantalum capacitors as opposed to aluminum electrolytic caps which were prevalent at the time.  Unfortunately there are no available ESR plots for the ADS78xx series of parts.