NRF52840 ADC noise

RE: NRF52840 ADC noise

Karl Ylvisaker — Tue, 13 Jul 2021 11:12:38 GMT

Hello,

I am glad to hear that you are able to reduce the noise you are seeing when switching to Micropython, though I have never heard about Micropython personally.
Are you now measuring the reference voltage with an oscilloscope?

Enabling burst and oversampling should decrease the effects of noise on your output, since it then takes multiple samples in a row and averages them before outputting a single sample.

[quote user="Mtk"]The mainstream micropython ADC implementation adc.c is not using oversampling, calibration[/quote]

It is recommended to perform calibration upon initializing the SAADC, and whenever the ambient temperature has changed by more than 10 C since the last calibration.

[quote user="Mtk"]… As well as I was not able to find more advanced example configurations. Wondered if you could recommend some ?[/quote]

I am not exactly sure what I should point you to when you ask for more advanced configurations, but please have a look at this repository for multiple SAADC examples using different configurations.

Best regards,
Karl

RE: NRF52840 ADC noise

Mtk — Mon, 12 Jul 2021 04:58:22 GMT

Switched to Micropython, as well as disabled the int. DC-DC regulator and used the following settings:

.burst = NRF_SAADC_BURST_ENABLED

…..

nrfx_saadc_simple_mode_set((1 << adc_obj->id), NRF_SAADC_RESOLUTION_14BIT, SAADC_OVERSAMPLE_OVERSAMPLE_Over8x, NULL);

The results (measuring 1.406 V) seems better – stdev = 4. The mainstream micropython ADC implementation adc.c is not using oversampling, calibration … As well as I was not able to find more advanced example configurations. Wondered if you could recommend some ?

RE: NRF52840 ADC noise

Karl Ylvisaker — Tue, 22 Jun 2021 07:59:39 GMT

Hello,

[quote user="Mtk"]With 47uF capacitor on the input ? Using the same LDO with STM32WB55 I get about 11 ENOB in 12-bit mode (no oversampling), deviation about 1 LSB. In the nrf52840 case what is the measured ENOB – 8 ?[/quote]

Unfortunately you must expect a couple of LSB deviation with the SAADC. Please see the replies by my colleagues in this and this ticket for more information.
However, I would also say as @awneil already has; A multimeter is not suited to measure/identify noise, because of its averaging. Do you have access to an oscilloscope with which you could perform these measurements?

[quote user="Mtk"]Have some questions: I was trying different boot loaders including open_bootloader from nRF5_SDK_17.0.2_d674dde.[/quote]

You likely will have to make some modifications to make it work with your CDSENET module, thought I have no personal experience with this exact module.
I would ask that you open a separate ticket for the bootloader issues since it diverges from the original ticket, in order to keep the forum tidy and easy to navigate for other users.

[quote user="Mtk"]Also, is there any ready firmware that can use nrf52840 as regular Bluetooth adapter for testing other BLE devices etc. ?[/quote]

Yes, please check out the pc-ble-driver and pc-ble-driver-py. These will let you create a desktop application using either cpp or python.

Best regards,
Karl

RE: NRF52840 ADC noise

Mtk — Mon, 21 Jun 2021 11:35:15 GMT

With 47uF capacitor on the input ? Using the same LDO with STM32WB55 I get about 11 ENOB in 12-bit mode (no oversampling), deviation about 1 LSB. In the nrf52840 case what is the measured ENOB – 8 ?

Have some questions: I was trying different boot loaders including open_bootloader from nRF5_SDK_17.0.2_d674dde. Non of them worked, except Adafruit_nRF52_Bootloader. With Nordic tools get errors like:

Failed to upgrade target. Error is: read failed: device reports readiness to read but returned no data (device disconnected or multiple access on port?)

Any specifics ?

Also, is there any ready firmware that can use nrf52840 as regular Bluetooth adapter for testing other BLE devices etc. ?

RE: NRF52840 ADC noise

awneil — Mon, 21 Jun 2021 09:36:42 GMT

[quote userid="105424" url="~/f/nordic-q-a/76444/nrf52840-adc-noise/315958#315958"]a multimeter measured a stable value 120 mV, which it is a prove that the noise is in uV region[/quote]

Not really.

A multimeter will do a lot of averaging.

RE: NRF52840 ADC noise

Mtk — Thu, 17 Jun 2021 17:01:29 GMT

The input voltage is supplied from ultra-low noise LDO (TPS79933 – output noise voltage 34.65 uV). Also, I have a 47 uF capacitor on the input, as you can see from the picture it is high-frequency noise. In addition, a multimeter measured a stable value 120 mV, which it is a prove that the noise is in uV region. The measures from the internal ADC are in the range: 114.947 mV - 123.851 mV (-5.053 mV to +3.851 mV deviation from the average measure), which I consider not acceptable for the intended application.

RE: NRF52840 ADC noise

awneil — Thu, 17 Jun 2021 08:12:22 GMT

[quote userid="105424" url="~/f/nordic-q-a/76444/nrf52840-adc-noise"]Unfortunately the ADC readings are quite noisy[/quote]

How do you know it's not just your input signal that's noisy?

Your readings seem to be around 2375 +/- 75; ie, a variation of ~ 3% - which is actually not so bad, unless you've taken very great care over your analogue front-end, supply, layout, grounding, shielding, etc ...