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I designed a board that uses the RP2040 Pico for its ADC and I/O.

To improve the ADC of RP2040 I wanted to use an external reference. In the past, for other projects, I used as external reference a LT1019 (a shunt or series reference) and it worked well.

For this new design, I decided to use a precision series reference (REF5030) and I overlooked the datasheet which specified you should use a shunt reference... (as a LM4040).

Circuit series reference -> RP2040 Pico

When I switched ON the board, I started to get a lot of noise at the output of the REF5030. By adding a resistor in series (200 ohm) to the output of the REF5030, the instability at the output of the REF5030 decrease to 20 mV (from 100 mV).

I went through the circuits board provided in the RP2040 Pico datasheet, in the appendix at p.24 I got the wiring of the ADC_VREF.

Since it will be difficult (and expensive) to re-build the board that I made in a reasonable amount of time, I was wondering if I could change R9 (1 ohm) on the Pico board (see image attached) to an higher value resistor.

Resistor on the RP2040 board

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It should be okay with 1Ω according to the reference datasheet, assuming a ceramic capacitor.

However, it's not clear what your actual schematic is. If you have C14 as shown in the top schematic, you should remove that or replace it with a tantalum capacitor.

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  • \$\begingroup\$ Hi Spehro, thank you for your message. In my schematic, I have a ceramic capacitor of 1uF (C14) at the output of the REF5030, as recomended in the datasheet of the REF5030. While I could see that removing the capacitor might remove the oscillation, why would a tantalum capacitor be a better? \$\endgroup\$
    – Kostr
    Aug 19, 2023 at 19:19
  • \$\begingroup\$ Hi Spehro, thank you for your message. Good point, the capacitor should have low ESR to maintain stability. I planned to get a low ESR capacitor, but while ordering I forgot about. \$\endgroup\$
    – Kostr
    Aug 19, 2023 at 19:29
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    \$\begingroup\$ Not just low ESR, ESR in the range of 1.0 to 1.5Ω as per the datasheet. C14 may have too low ESR if it is a ceramic type so it could cause oscillation or ringing. \$\endgroup\$ Aug 20, 2023 at 3:29

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