[Urgent] Extenal reference voltage for Nordic ADC

[MarkZhang]

Hello,

We have a project which requires high accuracy for 14-bit resolution ADC output by using Nordic nrf52832, we regard that the accuracy is linked to the internal reference voltage 0.6v that we had used in the firmware.

The current output accuracy of power supply for Nordic Vdd is 2.8v +/- 2%, we're using internal reference voltage.

Here are several questions, can you help me out?

1. What's the accuracy level of Nordic nrf52832 internal reference voltage - 0.6v?  

    After checking from the blog, we find some answers, like +/- 3%, +/- 0.2%, which one is correct?

    If both of them are incorrect, what's the correct tolerance of accuracy?

2. If we want to achieve more accurate reference voltage, can we use a more accurate Vdd supply?

    Would the accuracy of Vdd be impacted by Nordic internal LDO (or DC/DC)?

    If we use a high accurate external power supply for Vdd, like 3v (+/- 0.08%), can I get 0.75v (+/- 0.08%) (Vdd/4 mode) as reference voltage for ADC?

 

3. Is there another way that we could use external reference voltage to achieve a high accuracy of reference voltage?

    For example, it seems like external reference can be used for comparator; can we use "Analogy input port" to achieve reference voltage with high accuracy for ADC?

4. Does Nordic have any recommended solution to achieve 14-bit resolution of ADC in a stable and accurate manner for nrf52832 from hardware and/or software aspects?

    Any recommendation on the accuracy level of power supply to achieve 14-bit resolution on nrf52832?

[Håkon Alseth]

Hi,

 

1. What's the accuracy level of Nordic nrf52832 internal reference voltage - 0.6v?  

    After checking from the blog, we find some answers, like +/- 3%, +/- 0.2%, which one is correct?

    If both of them are incorrect, what's the correct tolerance of accuracy?

It is not directly stated in the PS, but the spec is +/-3% over the whole temperature range, and typically +/- 0.2% at room-temp.

2. If we want to achieve more accurate reference voltage, can we use a more accurate Vdd supply?

    Would the accuracy of Vdd be impacted by Nordic internal LDO (or DC/DC)?

    If we use a high accurate external power supply for Vdd, like 3v (+/- 0.08%), can I get 0.75v (+/- 0.08%) (Vdd/4 mode) as reference voltage for ADC?

If you set VDD as a reference, the reference will be as accurate as what it is sourced from. I would recommend that you test with both external and internal source to see the difference between the two.

3. Is there another way that we could use external reference voltage to achieve a high accuracy of reference voltage?

    For example, it seems like external reference can be used for comparator; can we use "Analogy input port" to achieve reference voltage with high accuracy for ADC?

REFSEL is either internal or VDD/4.

Given the specs that you posted in your initial reply, I assume you have already read this thread? There are some good discussion here related to ADC integration and testing:

 nRF52833 ADC reference external interface 

4. Does Nordic have any recommended solution to achieve 14-bit resolution of ADC in a stable and accurate manner for nrf52832 from hardware and/or software aspects?

    Any recommendation on the accuracy level of power supply to achieve 14-bit resolution on nrf52832?

The effective number of bits (ENOB) is 9 for the SAADC:

https://docs.nordicsemi.com/bundle/ps_nrf52832/page/saadc.html#d944e45805

 

This can be improved to approx. 10.5 if using OVERSAMPLING=4x, as described in this thread:

 RE: nRF52 ADC ENOB 

And always ensure that you are doing an offset calibration by triggering task TASKS_CALIBRATEOFFSET on boot-up and if your environment changes.

 

What are your requirements in terms of ENOB and sampling rate? If you need ENOB>10.5 bit, then I would recommend looking at external ICs for AD converting.

 

Kind regards,

Håkon

 

[MarkZhang]

Thanks for your quick response.

We are planning to use a more stable power supply for Vdd of Nordic 52832, by doing so, we can compare the difference between internal reference and Vdd/4. I will let you know once we have a test results.