SAADC calibration on nRF52840

RE: SAADC calibration on nRF52840

hmolesworth — Tue, 11 Feb 2025 16:43:45 GMT

Pity, but yes you can still improve the readings. A lot depends on the board layout, and whether the unused AIN pins are attached to anything. I would start by taking 2 adjacent spare analogue inputs and enabling both high and low 160k internal bias resistors then take a differential reading with the two inputs in 1-pin_p 2-pin_n format then reverse and take a reading in 1-pin_n and 2-pin_p. The difference gives the 2 x the residual SAADC offset. The gain settings and reference need to be the same as used for the real data sampling. Take lots of readings, or use OVERSAMPLE.

Perhaps try repeating with internal both VDD/4 and internal both GND to see how they compare, again reversing p and n to get balanced values. I would expect the best results from using the mid rail bias however.

If there is only a single spare input pin this is not an option. Maybe share the front-end schematic and input voltage ranges in case I see another option.

RE: SAADC calibration on nRF52840

Rafael Ramos — Tue, 11 Feb 2025 15:06:02 GMT

Thanks for the quick reply.

The hardware in this case is already in the field in large numbers and we want to tune things to get extra quality out of the current setup, so I can't add resistors or tie pins externally to GND, etc.

I do have unused analog inputs so we can dedicate one to a calibration but can we tie it internally to gnd with firmware ? is there another idea you can suggest?

The input is a 0-5v sensor over a relatively long cable that is filtered and conditioned down to 3v on board and then fed to the NRF52840.

Thanks !

RE: SAADC calibration on nRF52840

hmolesworth — Tue, 11 Feb 2025 05:33:33 GMT

Calibration always results in a discontinuous jump in the baseline, and although calibration is worth doing initially is not very effective following the initial calibration. Far better to use a true differential measurement, even if one end of the measurement is effectively GND, and simply reverse the inputs for successive readings and take the average. Such a reversal is trivial and is performed in software, not hardware; 2 analogue inputs are required, even if one end is at GND; a single input will not allow residual offset correction, I posted a schematic (circuit diagram) and low-level (bare-metal) code showing how to do this for an RTD resistance measurement, getting near 12-bits ENOB which is pretty amazing for a general-purpose MPU internal ADC. Temperature tracking is automatic and smooth provided SAADC, reference resistor and sense resistor are all in close proximity..

Measured 14-bit value of 905.0 Ohm 2-Wire test resistor using 128 samples is 904.9 Ohms with -0.01% error

// Internal 0.6v Ref 14-bit S0S1 2-Wire RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Pos-Neg ADC
// VDD mV Iref uA  RdsOn  V1 mV V2 mV V3 mV    Vrtd    Vref   Rrtd  Error
// ====== ======= ======  ===== ===== =====  ======  ====== ====== ======
//   2974    1008     41   2932   911    -3  3108.5  6883.3  903.8 -0.13%
//   2975    1007     41   2933   912    -2  3108.3  6882.6  903.8 -0.13%
//   2974    1007     37   2936   913    -2  3107.9  6882.4  903.8 -0.14%
//   2976    1007     42   2933   914    -2  3108.5  6882.0  904.0 -0.11%
//   2973    1007     38   2934   911    -2  3108.3  6881.3  904.0 -0.11%
//   2977    1007     42   2934   911    -1  3110.3  6882.9  904.4 -0.07%
//   2978    1008     40   2937   913    -1  3110.1  6884.8  904.1 -0.10%
//   2977    1008     40   2936   913    -1  3109.8  6885.3  903.9 -0.12%
//
// Internal 0.6v Ref 14-bit S0S1 2-Wire RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Neg-Pos ADC
// VDD mV Iref uA  RdsOn  V1 mV V2 mV V3 mV    Vrtd    Vref   Rrtd  Error
// ====== ======= ======  ===== ===== =====  ======  ====== ====== ======
//   2977    1010     41   2935   912    -3  3120.1  6896.4  905.5  0.05%
//   2976    1009     40   2935   912    -2  3119.5  6893.8  905.7  0.07%
//   2973    1009     39   2933   912    -2  3119.4  6892.3  905.8  0.09%
//   2976    1010     40   2935   911     0  3121.9  6896.3  906.0  0.11%
//   2975    1010     39   2935   913    -2  3120.9  6896.3  905.7  0.08%
//   2975    1009     40   2934   911    -1  3120.0  6891.4  906.1  0.12%
//   2976    1009     41   2934   912     0  3118.6  6892.3  905.6  0.07%
//   2975    1009     38   2936   913    -1  3118.3  6891.4  905.6  0.07%
//
// Measured 14-bit value of 905.0 Ohm 2-Wire test resistor using 128 samples is 904.9 Ohms with -0.01% error

seeking-more-information-on-nrf52833-adc-reference

RE: SAADC calibration on nRF52840

Rafael Ramos — Tue, 11 Feb 2025 04:46:40 GMT

I have a very similar issue, but this suggestion is confusing, Should I configure both the negative and postive of the same channel to the same input at the same time ? (VDD)

I am assuming I would need to configure it to be differential?

What reference should I use at this point ? The internal one ?

I am not sure this makes sense to me, but I tried it and only got zero for the reading (as I would expect, but not what this post suggests)

This is the actual config code :

nrf_saadc_channel_config_t channel_config = {

.resistor_p = NRF_SAADC_RESISTOR_DISABLED,

.resistor_n = NRF_SAADC_RESISTOR_DISABLED,

.gain = NRF_SAADC_GAIN1_4,

.reference = NRF_SAADC_REFERENCE_INTERNAL,

.acq_time = NRF_SAADC_ACQTIME_40US,

.mode = NRF_SAADC_MODE_DIFFERENTIAL,

.burst = NRF_SAADC_BURST_ENABLED,

.pin_p = NRF_SAADC_INPUT_VDD,

.pin_n = NRF_SAADC_INPUT_VDD

};

I have tried super sampling and a filtering but the temperature effects can't really be accounted for , and every time the calibration runs, the values jump to a new baseline, so that ruins long term calculations.

I would appreciate some help in this regard.

Thanks !

RE: SAADC calibration on nRF52840

haakonsh — Tue, 07 Apr 2020 09:20:40 GMT

[quote user="harti"]Where can I find a functional description?[/quote]

I'm sorry but that's proprietary information.

[quote user="harti"]How big is the jump that occurs after a calibration?[/quote]

The calibration DAC is around +-2 LSB10b (+-8 LSB12b)

RE: SAADC calibration on nRF52840

haakonsh — Tue, 07 Apr 2020 09:16:14 GMT

[quote user="dzabel"]I should completely avoid the the built-in calibration and instead do a digital offset calculation as described in step 1-3?[/quote]

Yes, I recommend it for your application.

[quote user="dzabel"]what is happening exactly if I use the built-in calibration and how I can observe the calculated offset.[/quote]

The built-in calibration uses an analog reference to compensate for the offset, but this reference is susceptible to thermal noise that will give a slightly different offset each time.

RE: SAADC calibration on nRF52840

dzabel — Tue, 07 Apr 2020 08:41:29 GMT

Hi,

thank you for the reply. I was using HFXO already. I have activated oversampling as suggested, and it does reduce the noise a bit. But it does not influence the strange calibration behaviour, i.e. sudden unexpected shifts of the values after calibration task has been executed. I have tried to increase the calibration rate by doing it time-dependent instead of temperature-dependent, but that only seems to increase the rate of value shifts. So do I understand you right I should completely avoid the the built-in calibration and instead do a digital offset calculation as described in step 1-3? My question would be still what is happening exactly if I use the built-in calibration and how I can observe the calculated offset.

Regards Dirk

RE: SAADC calibration on nRF52840

harti — Tue, 07 Apr 2020 08:36:02 GMT

I find neither an indication of the temperature drift of the internal reference nor a description of how the calibration works.
Where can I find a functional description?
How big is the jump that occurs after a calibration?

RE: SAADC calibration on nRF52840

haakonsh — Fri, 03 Apr 2020 12:45:29 GMT

The analog reference used in the automatic offset calibration is a bit temperature-sensitive and not really suited in designs with high accuracy requirements.

What you can do is a digital offset calibration:

Connect positive and negative inputs to the same signal, f.ex VDD/4.
Take a sample.
Subtract the sample value from all future samples (until you calibrate again).

This will reduce the offset further and have less variation in offsets between calibrations, but it will require a float or integer operation extra.

I also strongly recommend that you enable oversampling, at least 4-8X as there can be a fair bit of transient noise in the power supply for the thermistor.

You can also turn on the HFXO for a further increase in accuracy.