[Urgent] Extenal reference voltage for Nordic ADC

RE: [Urgent] Extenal reference voltage for Nordic ADC

MarkZhang — Mon, 23 Dec 2024 03:10:26 GMT

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.

RE: [Urgent] Extenal reference voltage for Nordic ADC

hmolesworth — Thu, 19 Dec 2024 16:42:21 GMT

What are you sampling? There may be a way to use a ratiometric measurement mode; in this recent answer I achieved a measured value of 905.2 Ohms with 0.02% error on a 905.0 Ohm test resistor using 12-bit sampling (14-bit slightly better) using a noisy USB supply (even better using a battery supply). I included a schematic and my code.

nrf52833-adc-reference

// Internal 0.6v Ref 12-bit S0S1 RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Forward current measurement
//        VDD     Iref    RdsOnA    RdsOnB      V1      V2      V3      V4         Vrtd         Vref        Rrtd  Error
//     ======   ======  ========  ========  ======  ======  ======  ======  ===========  ===========  ========== ======
// RTD 2982mV,  926 uA,  173 Ohm,  136 Ohm, 2821mV, 1989mV, 1977mV,  126mV,  477 counts, 1055 counts, 904.9 Ohms -0.01%
// RTD 2979mV,  925 uA,  168 Ohm,  139 Ohm, 2823mV, 1984mV, 1984mV,  129mV,  477 counts, 1054 counts, 905.8 Ohms  0.08%
// RTD 2978mV,  926 uA,  174 Ohm,  150 Ohm, 2816mV, 1989mV, 1989mV,  139mV,  478 counts, 1055 counts, 906.8 Ohms  0.20%
// RTD 2981mV,  925 uA,  172 Ohm,  144 Ohm, 2821mV, 1989mV, 1989mV,  134mV,  477 counts, 1054 counts, 905.8 Ohms  0.08%

// Internal 0.6v Ref 12-bit S0S1 RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Reverse current measurement
//        VDD     Iref    RdsOnA    RdsOnB      V1      V2      V3      V4         Vrtd         Vref        Rrtd  Error
//     ======   ======  ========  ========  ======  ======  ======  ======  ===========  ===========  ========== ======
// RTD 2984mV,  922 uA,  135 Ohm,  183 Ohm,  125mV,  965mV,  970mV, 2815mV,  475 counts, 1051 counts, 904.5 Ohms -0.05%
// RTD 2980mV,  922 uA,  144 Ohm,  180 Ohm,  133mV,  960mV,  959mV, 2814mV,  475 counts, 1051 counts, 904.5 Ohms -0.05%
// RTD 2981mV,  923 uA,  144 Ohm,  179 Ohm,  133mV,  961mV,  965mV, 2815mV,  475 counts, 1052 counts, 903.7 Ohms -0.15%
// RTD 2979mV,  921 uA,  145 Ohm,  170 Ohm,  134mV,  970mV,  965mV, 2822mV,  475 counts, 1050 counts, 905.4 Ohms  0.04%

// Measured 12-bit value of 905.0 Ohm test resistor is 905.2 Ohms with  0.02% error

// Internal 0.6v Ref 14-bit S0S1 RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Forward current measurement
//        VDD     Iref    RdsOnA    RdsOnB      V1      V2      V3      V4         Vrtd         Vref        Rrtd  Error
//     ======   ======  ========  ========  ======  ======  ======  ======  ===========  ===========  ========== ======
// RTD 2976mV,  923 uA,  169 Ohm,  138 Ohm, 2820mV, 1979mV, 1972mV,  128mV, 1898 counts, 4205 counts, 903.4 Ohms -0.18%
// RTD 2973mV,  924 uA,  174 Ohm,  136 Ohm, 2812mV, 1975mV, 1977mV,  126mV, 1899 counts, 4211 counts, 902.6 Ohms -0.27%
// RTD 2968mV,  923 uA,  154 Ohm,  136 Ohm, 2825mV, 1989mV, 1978mV,  126mV, 1897 counts, 4209 counts, 902.0 Ohms -0.33%
// RTD 2973mV,  923 uA,  167 Ohm,  130 Ohm, 2818mV, 1983mV, 1978mV,  120mV, 1898 counts, 4209 counts, 902.5 Ohms -0.28%

// Internal 0.6v Ref 14-bit S0S1 RTD actual value 905.0 Ohm (reference 2001.4 Ohm) - Reverse current measurement
//        VDD     Iref    RdsOnA    RdsOnB      V1      V2      V3      V4         Vrtd         Vref        Rrtd  Error
//     ======   ======  ========  ========  ======  ======  ======  ======  ===========  ===========  ========== ======
// RTD 2968mV,  924 uA,  130 Ohm,  168 Ohm,  121mV,  954mV,  958mV, 2812mV, 1912 counts, 4213 counts, 908.3 Ohms  0.36%
// RTD 2973mV,  923 uA,  131 Ohm,  173 Ohm,  121mV,  964mV,  960mV, 2813mV, 1909 counts, 4209 counts, 907.7 Ohms  0.30%
// RTD 2979mV,  925 uA,  136 Ohm,  175 Ohm,  126mV,  959mV,  963mV, 2817mV, 1908 counts, 4216 counts, 905.8 Ohms  0.08%
// RTD 2976mV,  924 uA,  132 Ohm,  170 Ohm,  122mV,  953mV,  966mV, 2818mV, 1909 counts, 4214 counts, 906.7 Ohms  0.18%

// Measured 14-bit value of 905.0 Ohm test resistor is 904.9 Ohms with -0.01% error

RE: [Urgent] Extenal reference voltage for Nordic ADC

Håkon Alseth — Thu, 19 Dec 2024 13:06:10 GMT

Hi,

[quote user=""]

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?

[/quote]

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.

[quote user=""]

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?

[/quote]

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.

[quote user=""]

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?

[/quote]

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

[quote user=""]

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?

[/quote]

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