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,

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

Best regards,

Kiyoshi Iwai

[Kenneth]

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

 

 

 

[Kiyoshi Iwai]

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

[Kenneth]

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

 

 

[Kiyoshi Iwai]

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