oscillation at the embed analog front end circuits of SAADC in nRF52832

[Kiyoshi Iwai]

Hello,

I found the strange phenomena on nRF52832.

By using the SAADC, I have some noise on ADC Data only at a certain temperature range.

Temperature range is depend on individual IC.

I might be the circuit oscillation at analog front end of SAADC, I think.

(internal reference voltage or op amp etc,)

I tried to switch DCDC on/off, but there's no difference on phenomena.

Anyone can resolve it?

I attach the files of real data and measurement condition.

Best regards,

Kiyoshi Iwai

3426.SAADC_noise_devzone.zip

[Kiyoshi Iwai]

Hi Kenneth,

marking is following.

CIAA chips are "N52832  CIAAE1 1803AA" all.

QFAA chips are "N52832 QFAAB0 1740JK" and "N52832 QFAAB0 1701FH".

And why don't you use our project?

Our software engineer  took a long time (almost 3 days) to modify the project which can be submitted to other company. And Avnet also used this project  and confirmed to recreate this issue.

You are going to waste all things ?

I strongly recommend to compile our project, use our project and reproduce this issue at your Lab.

This problem is almost on Hardware obviously, I think.

Could you find the root cause of Hardware as soon as possible?

(And please return this to public mode.)

 

Best regards,

Kiyoshi Iwai

 

[Kenneth]

Hi Kiyoshi,

Thank you for providing the chip markings.

The reason for not using your project is that the test lab must perform the tests in a controlled manner, to ensure we can identify if this is a hardware issue. If we test using your software, then we will not know if it's caused by software or hardware. This is currently the highest priority task to find out.

The test lab are preparing test firmware, and hopefully the automated test can run during tonight, so we have results by tomorrow. That is the plan. I will let you know if there is any delay.

Best regards,
Kenneth

[Kiyoshi Iwai]

You mean 5us x 128 x 7168 =4.5sec ?  It's too short, I think.

Could you check again  with longer interval time( 30min-1 hour)at one temperature ,

and go next temperature.  You may use wider temperature step. (5 or 10 degree C)

Thanks,

Kiyoshi Iwai

[Kenneth]

Hi Kiyoshi,

If you can replicate the setup, then try the following:

- Use 2x AAA for supply (with short wires) for power supply

- Use 1x AAA for analog input . 

Thank you very much if you can do this.

Best regards,
Kenneth

[Kenneth]

Hi Kiyoshi,

Please do not do further testing at your side.

I believe our test lab have been able to repeat the measurements now. They experience some noise, and the noise have similar peak that you see, however they need to do further tests to confirm it is the same or something else. I will keep you updated when I know more.

Best regards,
Kenneth

[Kiyoshi Iwai]

Hi Kenneth,

That's the best information in a few weeks.

I do keep waiting for next information of root cause and counter measure.

Best regards,

Kiyoshi Iwai

[Kenneth]

Hi Kiyoshi,

The project manager is out travelling this week, but I have sent him an e-mail to inform him that we can see noise in the measurements, and asked him to suggest for further tests the lab can do to narrow down the source of the noise. 

I can only apologize for the time this is taking.

Best regards,
Kenneth

[Kiyoshi Iwai]

Hi Kenneth,

Do you have something updated?

I have to explain the current situation to other section in our company tomorrow.

Could you tell me the lab's result of the further tests , suggestion of the project manager and etc.,?

Best regards,

Kiyoshi Iwai

[Kenneth]

Hi Kiyoshi,

I believe our test lab are narrowing down the different causes. I can add a few of the measurements here:

Noise on ADC measurements due to noise on internal reference voltage (0.6V):

Noise on CRC2032 during sampling (128x bursts):

Cell voltage of CR2032 over temperature:

Both above measurements are without radio, which will cause even more noise.

As it looks now the recommendation will be to use external reference (VDD/4), but the external reference can't be a CR2032 directly.

My suggestion to confirm is to use 1xAAA (or 1x AA) for analog input, and 2xAAA (or 2x AA) for supply voltage (with short wires). The external crystal oscillator should also be started before measurements, since the internal RC oscillator is influenced by temperature.

Best regards,
Kenneth

 

 

 

 

 

[Kiyoshi Iwai]

Hi Kenneth,

Thank you for all your cooperation.

You mean that nobody can use the internal reference 0.6V with CR2032, right?

First of all, when I started to talk about this problem, I have pinned down the influence of temperature.

I have also good temperature range and bad temperature range at all same condition except for temperature.

If the cause were the temperature characteristic of CR2032, this issue shall occur at low temperature on all boards.

But the result was different. Some board has a failure on 25 degree C and other has it on 35 degree C.

And there are OK on -5 degree C and bad on 15 degree C on same board.

How do you explain that?

All causes are CR2032?  I don't think so at all.

I only want to operate the ADC with good performance at all temperature, because ADC has a low noise state  at a certain temperature range.

Could you please focus on only temperature difference and find the root cause of this issue soon?

I always greatly appreciate your kindness.

Best regards,

Kiyoshi Iwai

[Kenneth]

Hi Kiyoshi,

The noise in internal reference is temperature dependent, and likely the main cause of your issue.

The frequency of the internal RC oscillator is also temperature dependent, and will influence measurement.

You wrote you also experienced noise when using external reference(VDD/4), and we believe the cause of that is CR2032 is not linear over temperature, we can see that there is noise from CR2032 (VDD and analog input), the noise depends on current consumption (burst sampling) and temperature.

Best regards,
Kenneth

[Kenneth]

Kiyoshi Iwai said:

You mean that nobody can use the internal reference 0.6V with CR2032, right?

I think that will depend on the required accuracy of the ADC measurements, but for 12(14-bit with oversampling) you likely will not be able use the internal reference or a CR2032 directly no (but possibly with an LDO).

Best regards,
Kenneth