Maximum possible SAADC differential voltage between AN0 and AN1 pins

[valioiv]

Hi! I want to measure voltage from a small inductor connected differentially to AN0 (Vp potential to GND) and AN1 (Vn potential to GND) directly. And I'm thinking to properly scale its voltage down externally by resistors (if needed)!

My power supply of NRF5340 is VDD=1.8V. I have configured SAADC to use its internal Vref=0.6V and SAADC Gain=1/6, so I can expect I can measure as follows:

Vdiff = Vp - Vn = 1.8 - (-1.8) = +3.6V

Vdiff = Vp - Vn = -1.8 - 1.8 = -3.6V

So the maximum voltage peak-to-peak I can measure looks like to be Vpp=3.6V.

Am I right in my interpretation?

PS1: I assume in SAADC differential mode the Vp/Vn potential can be negative compared to ground!

PS2: None of the AN0 and AN1 is grounded!

[hmolesworth]

Here's an example of using a Flying Capacitor - Simplest single-phase, single SAADC Input. Can use two inputs and also multiple phase but this works pretty well and save the extra analogue input.

SPDT Switch, must be Break-before-Make; select one with both single-supply and negative input voltage capability, example max4765.pdf or higher input voltage MAX20327.pdf

// Flying Capacitor - Simplest single-phase, single SAADC Input
//
//                                            +----------------------------------------------+
//                                            |                 nRF52832                     |
//                           +------#---------0---< PWM                                      |
//                           |      |         |                                              |
//                           |      |         |    VDD                                       |
//                           |      |         |   --#--                                      |
//                           |      |         |     |                                        |
//                           |      |         |     |   Option                               |
//                           |      |         |     +-|   160k                               |
//                           |      |         |       |<- High                               |
//                           |      |         |     +-|   Bias                               |
//                           ~      |         |     |                                        |
//     +------#----------- 0   0--------------O-----#------------#------#--> P0.nn SAADC     |
//     |      |             \       |         |     |   Option   |      |    Ref = Int       |
//     |      |              \      |         |     +-|   160k  +++     |                    |
//     |      |              0      |         |       |<- Low   | |   -----                  |
//     @     +++             |      |         |     +-|   Bias  | |   -----                  |
//     @     | |           -----    |         |     |           +++     | 2.5pF              |
//     @     | |           -----    |         |     |            |1M0   |                    |
//     @     +++             | C    |         |   =====        =====  =====                  |
//     | L    | R            0      |         |    ===          ===    ===                   |
//     |      |              /      |         |     =            =      =                    |
//     |      |             /       |         |                                              |
//     +------#------------0   0--------------0-----+    <== Gnd or use 2nd SAADC Input      |
//                           ~      |         |     |                                        |
//                           |      |         |   =====                                      |
//                           +------+         |    ===                                       |
//                                            |     =                                        |
//                                            +----------------------------------------------+

[valioiv]

Thank you so much for your recommendations! I'll consider what you are sharing for the new design of the device.