SAADC noise issue with Bluetooth softdevice (nRF52840)

RE: SAADC noise issue with Bluetooth softdevice (nRF52840)

Sebastien DRI — Tue, 26 Apr 2022 09:32:29 GMT

Hi Jorgen,

Thank you again for your answer.

More generally, thank for high level support you show at Nordic.

I don't do any periodic offset calibration at all but we plan to further characterize acquisition chain offset to correct in a software way.

We don't check if offset samples corresponds with radio activity but we're pretty sure it does since it is the only difference between the twwo generated firmwares.

I will contact you in a short time after being coming back on that subject in order to confirm/or not that hypothesis based on the method you mentionned.

Thank you again.

RE: SAADC noise issue with Bluetooth softdevice (nRF52840)

Jørgen Holmefjord — Mon, 11 Apr 2022 13:48:23 GMT

Hi,

Thanks for posting the raw samples. It looks like you have a difference of 14 symbols between min and max sampled value, which corresponds to ~0.34% with 12 bit resolution. I think this is well within the specifications of the SAADC peripheral (there is a gain error of +/-3%, calibrated offset error of +/- 8 LSB12b, etc).

Are you running any (periodic) offset calibration in your application?

Have you checked if the "offset" samples corresponds with radio activity, etc? You can use the radio notification feature or the PA & LNA GPIO toggle feature to see when the radio is active. If you use this to toggle GPIOs on radio activity and do the same when the SAADC SAMPLE task is triggered, you can see the GPIO toggling using a logic analyzer, to see if the sampling overlaps with the radio activity.

Radio activity, flash activity, and high current consumption may cause more noise on the SAADC sampling.

Best regards,
Jørgen

RE: SAADC noise issue with Bluetooth softdevice (nRF52840)

Sebastien DRI — Mon, 04 Apr 2022 15:14:42 GMT

Hi Jorgen,

You'll find hereunder the updated spreadsheet with ADC raw data displayed (simple linear voltage=>current transformation).

devzone.nordicsemi.com/.../ADCCurrent_5F00_bis.xlsx

RE: SAADC noise issue with Bluetooth softdevice (nRF52840)

Jørgen Holmefjord — Fri, 01 Apr 2022 16:02:28 GMT

[quote user="sebastopolix"]

When you say "just by starting softdevice (+8dbm level selected)", do you mean that the softdevice is enabled, but not transmitting/receiving anything, i.e. no BLE roles enabled?

Yes, just advertising (Peripheral role) in long range mode with the extra +8dbm power emission. I run two measurement sessions with/without softdevice enabled.

[/quote]

Ok, that makes more sense than just enabling the Softdevice without any radio activity.

[quote user="sebastopolix"]No, I don't see any softdevice action to activate the DCDC regulator in our code. We use external LDO[/quote]

You can enable the DCDC regulator using the Softdevice API sd_power_dcdc_mode_set(NRF_POWER_DCDC_ENABLE), but the DCDC regulator will likely cause even more noise, so I was mainly asking if it was enabled.

[quote user="sebastopolix"]

Can you post the raw samples from the case with and without softdevice, to better see the difference?

Yes of course, hereafter are freshly new raw samples for a given current consumption on our board:

ADCCurrent.xlsx

[/quote]

The document seems to contain numbers with decimals, which I believe are somehow processed from the raw samples. Do you have the int16_t results form the SAADC buffer, without any processing/calculations?

Best regards,
Jørgen

RE: SAADC noise issue with Bluetooth softdevice (nRF52840)

Sebastien DRI — Thu, 31 Mar 2022 21:58:38 GMT

Hi Jørgen,

Sorry for the late reply;

When you say "just by starting softdevice (+8dbm level selected)", do you mean that the softdevice is enabled, but not transmitting/receiving anything, i.e. no BLE roles enabled?

Yes, just advertising (Peripheral role) in long range mode with the extra +8dbm power emission. I run two measurement sessions with/without softdevice enabled.

Do you have the DCDC regulator enabled in your application?

No, I don't see any softdevice action to activate the DCDC regulator in our code. We use external LDO

Here is also for information the Nordic schematic for the supply part:

You can consider enabling oversampling in the SAADC to reduce noise, but this will take more time, and should have given similar results to what you get if averaging manually.

Yes, for sure. In fact the second pattern noise pattern added by activating BLE acts as a second noise source apart from the histogram given with no BLE.

Can you post the raw samples from the case with and without softdevice, to better see the difference?

Yes of course, hereafter are freshly new raw samples for a given current consumption on our board:

devzone.nordicsemi.com/.../ADCCurrent.xlsx

Thank you again for your support.

The best.

Sebastien

RE: SAADC noise issue with Bluetooth softdevice (nRF52840)

Jørgen Holmefjord — Tue, 29 Mar 2022 13:49:03 GMT

Hi,

When you say "just by starting softdevice (+8dbm level selected)", do you mean that the softdevice is enabled, but not transmitting/receiving anything, i.e. no BLE roles enabled?

If you have roles enabled, do you sample the SAADC during transmission, or between BLE events?

Do you have the DCDC regulator enabled in your application?

You can consider enabling oversampling in the SAADC to reduce noise, but this will take more time, and should have given similar results to what you get if averaging manually.

Can you post the raw samples from the case with and without softdevice, to better see the difference?

Best regards,
Jørgen