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What is the purpose of the resistor ladders in the SAADC?

Elias

The documentation for the nRF52840's SAADC mention optional resistor ladders. What are they for? I see that the documentation specifies that the ladder's resistance is 160kΩ, and that it can be pulled up and down. Are these just pull-up/down resistors? Why would I want this?

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    Usual reason is to get a mid-rail bias when measuring an AC-coupled input signal (eg via a series capacitor). There are other creative use cases as well; the simple answer is that external resistors for these use cases can be avoided reducing BOM count. Even though the 160k value is imprecise, the two resistors (actually FETs) are in fact closely matched as they are on a single die at the same temperature with the same aspect ratio; they can in some cases provide a better mid-rail reference than external precision resistors provided the use of internal Gnd and Vdd is acceptable.

    See adc-negative-voltage

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    Usual reason is to get a mid-rail bias when measuring an AC-coupled input signal (eg via a series capacitor). There are other creative use cases as well; the simple answer is that external resistors for these use cases can be avoided reducing BOM count. Even though the 160k value is imprecise, the two resistors (actually FETs) are in fact closely matched as they are on a single die at the same temperature with the same aspect ratio; they can in some cases provide a better mid-rail reference than external precision resistors provided the use of internal Gnd and Vdd is acceptable.

    See adc-negative-voltage

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