RE: nRF54L15 ADC Rsource value

hmolesworth — Fri, 07 Feb 2025 03:13:24 GMT

Edit: Yes, Rsource as described in the documentation will be 666Kohm, but as ketiljo suggests adding a 100nF capacitor across R428 to Gnd will effectively reduce this so a faster sample time can be used.

Discussion:

R2 (or R428 in the first post) will be in parallel with RsourceSAADC. The specification refers to Rsource when choosing the sampling time, but Rsource in the documentation is a different animal altogether and represents the external resistance R1||R2 (ie R428||R429), not the SAADC internal resistance.

RsourceSAADC (the value you seek) is neither specified nor constant, but is measurable. It will (one day) be quoted as >1M0, similar to (say) the nRF52833. However, it is temperature dependent, and probably day-of-week dependent as well.

Ratiometric measurements are the key; do not remove R2 (ie R428) but dynamically calibrate the effect of RsourceSAADC on R2 (ie R428) by measuring a second known voltage and hence remove the value of RsourceSAADC from the result. The second known voltage can be introduced by (for example) connecting another resistor to Gnd in parallel with R2 with a spare open-drain output pin (H0D1) and comparing the SAADC results.

Here is a different circuit which illustrates the internal connections, for ratiometrically calculating resistance - the internal nRF54L15 circuit is pure speculation of course :-)



data-mode="c_cpp">// RTD Measurement - 2- and 3-wire Unidirectional current + ----------------------------------------------------------------------+ |  nRF52832/nRF52833/nRF52840/nRF54L15                                  | | |    VDD               VDD                 VDD                          | |   --#--             --#--               --#--                         | |     |                 |                   |                           | |   __|__               |   Excitation      |   Option                  | |    / \                +-|   On            +-|   13k                   | |   /-+-\                 |<- H0   H1         |<- Pullup                | |     |                 +-|   160R 35R      +-|   (not used)            | |                   |                           | -----#------#----------#-------------------#------------ In            | |     |      |          |   Excitation      |   Option                  | |   __|__  -----        +-|   Off           +-|   13k                   | |    / \   -----          |<- L0   L1         |<- Pulldown              | |   /-+-\    |3pF       +-|   160R 35R      +-|   (not used)            | |     |      |          |                   |                           | |   =====  =====      =====               =====                         | |    ===    ===        ===                 ===                          | |     =      =          =                   =                           | | |    VDD                                         VDD                    | |   --#--                                       --#--                   | |   __|__                    Auto-adjust:         |   160k              | |    / \                     Bias inputs at VDD/2 +-| Bias Hi/Lo/Both   | |   /-+-\                     VDD1_2                |<-                 | |     |   PIN_VOLTAGE_1                           +-|                   | |   AIN4          |----'T'-Mux-P---|        |            P SAADC  | -----#------#-------#--+ +----#-------+ +--#-----#------#------#-->    | |     |      |       |  | |    |       | |  |     |      |      |       | |   __|__  -----     |  ----   +-|    ----- |     +-|   +++     |       | |    / \   -----     |   |       |<--+  |   |       |<- | |   -----     | |   /-+-\    | 3pF   |   |     +-|   |  |   |     +-|   | |   -----     | |       |   +---- |--------#   |     |     +++     | 2.5pF | |   =====  =====     |         |     |  |   |     |      |1M0   |       | |    ===    ===      |       =====   |  |   |   =====  =====  =====     | |     =      =       |        ===    |  |   |    ===    ===    ===      | |         =    Select  |     =      =      =       | |                      |                           | |                      |    VDD                    | |  |----'T'-Mux-N---|  |   --#--                   | +--+ +----#-------+ +----+   |   160k              | | |   +-| Bias Hi/Lo/Both   | PIN_VOLTAGE_2                     | |     |<-                 | AIN5          |----'T'-Mux-----|  | |   +-|                   | -----#------#-------#--+ +----#-------+ +--# |   |            N SAADC  | +--+ +----#-------+ +----#---#------#------#-->    | | |   |      |      |       | | |   +-|   +++     |       | Note V3 can be negative          | |     |<- | |   -----     | PIN_VOLTAGE_3                    | |   +-|   | |   -----     | AIN7         |----'T'-Mux-----|  | |   |     +++     | 2.5pF | -----#------#-------#--+ +----#-------+ +--# |   |      |1M0   |       | +--+ +----#-------+ +----# =====  =====  =====     | | |  ===    ===    ===      | | |   =      =      =       | | |                         | | |   P: Single-ended       | | |   P&N: Differential     | PIN_VOLTAGE_0                     | |                         | |     |   AIN_VDD       |----'T'-Mux-----|  | |                         | |     +-----#--------#--+ +----#-------+ +--+ |                         | +--+ +----#-------+ +----#                         | |                         | |    GND                |----'T'-Mux-----|    |                         | -----#-----------------+ +----#-------+ +----+                         | |     |                                                                 | |   =====                                                               | |    ===                                                                | |     =                                                                 | +-----------------------------------------------------------------------+ = (3pF+2.5pF)*2^14=80nF voltage V1 and V2 in SAADC counts voltage V2 and V3 in SAADC counts

RE: nRF54L15 ADC Rsource value

Poki Huang — Fri, 07 Feb 2025 01:43:38 GMT

Any feedback about my latest question?

Thank you

Poki

RE: nRF54L15 ADC Rsource value

Poki Huang — Wed, 05 Feb 2025 04:06:14 GMT

By your provided link (https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/measuring-lithium-battery-voltage-with-nrf52 )

Rsource value only take R1, no R2

So I have one question, if we only mount R2, R1 un-mount, what is the Rsource value?

Thank you

Poki

RE: nRF54L15 ADC Rsource value

ketiljo — Tue, 04 Feb 2025 13:58:25 GMT

[quote user=""]As below pic., P1.07/AIN3 input voltage will be 0.6V (1.8V divider by R429 & R428)[/quote]

Yes

[quote user=""]So the Rsource value will be R429//R428 value(~666Kohm) or R429+R428 value(3Mohm)?[/quote]

Take a look at this: https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/measuring-lithium-battery-voltage-with-nrf52

When using large resistors in the voltage divider, you will load the divider when the ADC samples. You can use a buffer capasitor that will be charged to the divider level to avoid this.