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

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

Hung Bui — Wed, 18 Jul 2018 08:13:59 GMT

Hi Kiyoshi,

Kenneth is on vacation. He will be back in about 2 weeks. I'm sorry for the delay in our response.

Best Regards,

Hung

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

Kiyoshi Iwai — Wed, 11 Jul 2018 10:48:48 GMT

Hi Kenneth,

I heard you Nordic Semi has abandoned to investigate the root cause of this issue from Avnet today.

That shouldn't happen.

Why don't you want to know the root cause? I can't believe it.

Your own IC has obviously the fault and you Nordic Semi shall make a improvement.

If you were an engineer, you ought to demand the root cause naturally, I think.

Are there no engineer who want to find out the truth in Nordic Semi?

If so, you Nordic Semi will never to be able to become the IC supplier.

The IC supplier should take a responsibility for supplying the higher quality products to all customers.

When the customer point out the problem, the supplier has to investigate the root cause and answer the counter measure to the customer soon.

I hope you will re-start the investigation and find the root cause out in a short time.

First of all, I strongly need the root cause, you know.

Best regards,

Kiyoshi Iwai

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

Kenneth — Thu, 05 Jul 2018 07:58:33 GMT

Hi Kiyoshi,

I know the test lab are doing some measurements to understand the noise, but to answer your questions and summarize:

The auto-zero loop is internal design, which I believe zero the circuitry between each sampling. If there is noise here it could be accumulated. Regarding CALIBRATEOFFSET I have seen in my own measurements (using your firmware) that sometimes the calibrate offset can also cause an error similar to what you see, in that case 12 samples in a row have an offset until for instance next calibration occur.

There is a temperature dependency on the error, this is also something we have stated in the datasheet:

So I am sorry to say, but we likely can't meet your requirement in this case if you need 12bit accuracy over temperature. I can only apologize for the time this has taken.

Best regards,
Kenneth

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

Kiyoshi Iwai — Thu, 05 Jul 2018 03:09:43 GMT

Hi Kenneth,

Do you have any progress?

I really want to know you Plan.

BTW, what is the auto-zero loop?

Is that same thing with CALIBRATEOFFSET?

Best regards,

Kiyoshi Iwai

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

Kenneth — Mon, 02 Jul 2018 09:02:45 GMT

Hi Kiyoshi,

I assume you also have a deadline.

The team are still investigating and doing measurements, but it may seem that there is some short burst noise in the SAADC peripheral that is device specific and temperature dependent. The error is about 0.2% of full scale measurement when it occurs. There are two hypothesis of root cause:

- Accumulated burst noise in the auto-zero loop could cause an error.

- Burst noise on the reference buffer signal could potentially cause noise as we see here.

They plan to do a bit more measurements, but it is not likely there is a workaround to the problem you are experiencing.

I notice you have a few questions on the internal IP, I don't know the answers to those, typically this is not something we share.

I can only apologize for the time this has taken.

Best regards,
Kenneth

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

Kiyoshi Iwai — Fri, 29 Jun 2018 09:34:41 GMT

Hi Kenneth,

Do you have a plan untill when you find the root cause?

Best regards,

Kiyoshi Iwai

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

Kiyoshi Iwai — Fri, 29 Jun 2018 09:32:47 GMT

Hi kenneth,

Is this the final conclusion of you Nordic Semiconductor?

I have a big disappointment to read this.

Do you intend to terminate the investigation anymore?

By the way,

Could you tell me what power source do you use on the circuit of internal reference 0.6V?

Do you connect VDD to internal reference directly?

Or via the embedded LDO?

I have no block diagram of internal reference voltage, so could you provide it in detail?

Best regards,

Kiyoshi Iwai

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

Kiyoshi Iwai — Fri, 29 Jun 2018 09:26:04 GMT

Hi Kenneth,

What is the noise of CR2032? You mean CR2032 can make such higher frequency noise?

Unbelievable.

If the cause is the internal resistance of CR2032 as you said, there will be disadvantage at lower temperature.

The lower temperature, the more issue will occur.

Of course the characteristic of internal resistance for temperature is not linear, but it will not go up and down. It will go in one direction gradually.

I alread found that 15 degree is bad, but -5 degree is OK, or only 35 degree is bad, but other all lower temperatures are OK.

I don't think the cause is CR2032. Even though, you doubt CR2032?

Best regards,

Kiyoshi Iwai

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

Kenneth — Thu, 28 Jun 2018 13:09:50 GMT

Hi Kiyoshi,

It seems that we still measure noise using all the suggested improvements.

We will need to investigate further.

Best regards,
Kenneth

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

Kenneth — Thu, 28 Jun 2018 09:14:34 GMT

[quote user="kiyoshi_iwai"]You mean that nobody can use the internal reference 0.6V with CR2032, right?[/quote]

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

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

Kenneth — Thu, 28 Jun 2018 07:11:02 GMT

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

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

Kiyoshi Iwai — Thu, 28 Jun 2018 01:43:37 GMT

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

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

Kenneth — Wed, 27 Jun 2018 18:04:38 GMT

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

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

Kiyoshi Iwai — Wed, 27 Jun 2018 10:04:06 GMT

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

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

Kenneth — Tue, 26 Jun 2018 09:46:06 GMT

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

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

Kiyoshi Iwai — Tue, 26 Jun 2018 01:07:33 GMT

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

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

Kenneth — Mon, 25 Jun 2018 15:14:53 GMT

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

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

Kenneth — Mon, 25 Jun 2018 10:19:28 GMT

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

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

Kiyoshi Iwai — Mon, 25 Jun 2018 10:00:08 GMT

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

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

Kiyoshi Iwai — Mon, 25 Jun 2018 09:39:17 GMT

"If you test the same board multiple times, does it always happen on the identical temperature?"

=> Yes.

"Can you share boards you know is failing with us?"

=> What do you mean? You need our board? But Your DK has same issue too, so you can

find the problem by using DK.

And I will check with 2xAAA, but you already design your DK which you intend to operate with CR2032?

Thanks

Kiyoshi Iwai

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

Kenneth — Mon, 25 Jun 2018 09:21:23 GMT

Hi Kiyoshi,

7168 samples are done at each temperature, all measurements are 14-bit (128x oversampled). The lab use single AA as analog input.

Best regards,
Kenneth

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

Kiyoshi Iwai — Mon, 25 Jun 2018 09:14:41 GMT

Hi,

How long did you stay at one temperature step on automatic test?

(How many AD samples in one step?)

And is this data on the oversampling mode? (14bit output)

Thanks

Kiyoshi Iwai

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

Kenneth — Mon, 25 Jun 2018 09:12:43 GMT

[quote user="kiyoshi_iwai"][/quote]

And when i tried using the VDD/4 as reference voltage instead of internal 0.6V,

I supplied power to the boards by normal power-supply (3.0V).

But I still had this issue by using normal power-supply, not CR2032.

If you have long wires for the power supply, I would expect you could see the same. So I want to ask nicely that you try:

Use 2x AAA for supply (with short wires) and 2x AAA for analog input (or a single AAA is likely easier). Make sure power supply is connected directly to VDD without any diode in reverse direction.

Another questions:

- If you test the same board multiple times, does it always happen on the identical temperature?

- Can you share boards you know is failing with us?

Best regards,
Kenneth

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

Kenneth — Mon, 25 Jun 2018 08:39:35 GMT

Hi,

Some results from the lab:

Here is for instance the variation at 40degree C (manual measurement):

The peak to peak deviation is about 35 LSB.

If we plot the deviation as function of temperature (sweep -25 to +65C with 2C steps) the result (automatic test):

As you can see the peak to peak deviation is pretty stable around 30-40 LSB peak to peak deviation. They are not able to see any significant noise peaks.

Best regards,
Kenneth

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

Kiyoshi Iwai — Mon, 25 Jun 2018 06:39:50 GMT

Hello Kenneth,

Thank you for information.

At what temperature did you do this experiment?

At All temperatures you have this result in the case of CR2032?

This data is not same our phenomenon, I think.

Our case occurs only at a certain temperature range.(not all temperatures)

And when i tried using the VDD/4 as reference voltage instead of internal 0.6V,

I supplied power to the boards by normal power-supply (3.0V).

But I still had this issue by using normal power-supply, not CR2032.

According to your explanation, the internal resistance of CR2032 may affect to this issue.

But the internal resistance of CR2032 is several ohms at max., so the voltage drop will be dozens mV when TX/RX.

I don't think this drop may be a dominant.

By the way, could you tell me what does the test lab team check now concretely?

Do they check by changing temperature?

Best regards,

Kiyoshi Iwai