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

[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

[Kenneth]

Hi Kiyoshi,

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

We will need to investigate further. 

Best regards,
Kenneth

[Kiyoshi Iwai]

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

[Kiyoshi Iwai]

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