Correct calibration of the nRF51 ADC to correct offset and gain error

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

PhilipSTT — Mon, 12 Jun 2023 06:53:51 GMT

Hi Jared,

no new news at the moment, I was off the last days. I will try to get some new results until next week.

Thanks,

regards
Philip

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

Jared — Tue, 06 Jun 2023 14:22:16 GMT

Hi Philip,

Did you have any new results?

[quote user="PhilipSTT"] I think I would have to exchange nrF devices
from us to the Devkit to get reliable results in this test.[/quote]

Ok, you so you have the option of soldering off the nRF on the DK and then solder on a failing device? That would be a very good test 🙂

regards

Jared

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

PhilipSTT — Tue, 30 May 2023 15:36:38 GMT

Hi Jared,

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/428228"]What limits did you use?[/quote]

For Limits we are already pretty wide open, 5mv to 30mV for a 23mV signal (because lower
values are not critical for us, we can accept very low measurements as well)

Here you can see the bell curve for around 200k. measurements taken, a lot of them are
out of Limit (under 5mV or over 30mV)

drive.google.com/.../view

I cannot tell the exact values of all 700 fail PCBAs, but I have around 30pcs. here to play with.
What I can say is that nrF51 devices with lower OFFSET and GAIN error values
written to the memory are always more accurate, with or without error calibration by code.

Which also can be seen on this measurement taken last week:

[quote userid="87058" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/427285"](Taken with the 0x10000024: FFFF0105 (GAIN=1, OFFSET=5) - device)

FLUKE TRMS = ~503mV

Measurement A) = 510mV (DIFF.=7mV)
Measurement B) = 512mV (DIFF.=9mV)
Measurement C) = 513mV (DIFF.=10mV)
Measurement D) = 511mV (DIFF.=7mV)

If I do this measurement with the 0x10000024: FFFF0001 ((GAIN=0, OFFSET=1) - device:

FLUKE TRMS = ~503mV

Measurement A) = 505mV (DIFF.=2mV)
Measurement B) = 506mV (DIFF.=3mV)
Measurement C) = 506mV (DIFF.=3mV)
Measurement D) = 505mV (DIFF.=2mV)[/quote]

I can try to do measurements with more of this devices?
This should bring up a lot of different OFFSET & GAIN errors stored to the devices memories.

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/428228"]It would be nice if you were able to reproduce this with development kit, that way we could rule out any issues specific to your HW. [/quote]

I surely can try with the Devkit, but if the devkits nrF device has low OFFSET & GAIN error
it will (probably) also create more reliable results. I think I would have to exchange nrF devices
from us to the Devkit to get reliable results in this test.

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/428228"]Also, it's worth noting that nRF51 is a fairly outdated device, and several changes were applied for the nRF52 series when designing the new ADC, it was improved on several areas including introducing an internal calibration routine for the ADC. [/quote]

Sure yes, it's already fairly outdated. But was always a very good option for us (we are using it since a decade at least).

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/428228"]How is the sampled values for the 700 pcs, are they consisted with each other and closer to the real value without calibration? [/quote]

They are consistent if we are talking about ADC measurements, because if it has a higher ADC output
(positive offset) it has it on all ADC readings (we are doing 5 different ADC measurements while testing)
I can try this on the 30pcs. mentioned above, but will take some time.

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/428228"]Are you testing this in the same temperature?[/quote]

This could be something to consider, but as stated above, it correlates with the OFFSET and GAIN
error values stored in memory. But I will test this as well, let's see which effect it has on the device.

best regards
Philip

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

Jared — Tue, 30 May 2023 14:36:49 GMT

Hi Philip,

[quote user="PhilipSTT"]Yes of course, also our FLUKE devices have some inaccuracy, but they are "freshly" calibrated and I have compared the measured values between FLUKE 8846A, 189 & 87 V - the differences are marginal.[/quote]

Ok,

[quote user="PhilipSTT"]Can't tell if the other 99300pcs. are shifting as well, but they have been at least inside limits we defined for testing.[/quote]

What limits did you use?

It would be nice if you were able to reproduce this with development kit, that way we could rule out any issues specific to your HW.

Also, it's worth noting that nRF51 is a fairly outdated device, and several changes were applied for the nRF52 series when designing the new ADC, it was improved on several areas including introducing an internal calibration routine for the ADC.

How is the sampled values for the 700 pcs, are they consisted with each other and closer to the real value without calibration?

Are you testing this in the same temperature?

regards

Jared

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

PhilipSTT — Fri, 26 May 2023 05:56:51 GMT

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/427538"]I think it's difficult to evaluate these results as the FLUKE itself has some inaccuracy and it's difficult to tell what the actual voltage is. But it's clear that the calibration seems to shift the measurement from the average between the A, B, C, D.[/quote]

Yes of course, also our FLUKE devices have some inaccuracy, but they are "freshly" calibrated and I have compared the measured values between FLUKE 8846A, 189 & 87 V - the differences are marginal.

Something is wrong with calibration, calibration calculation or the devices itself. Out of 100k PCBAs we got around 700pcs. with this problem. Can't tell if the other 99300pcs. are shifting as well, but they have been at least inside limits we defined for testing.

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/427538"]This is much better, and I agree that it shows that the results without the calibration is better in this case and more consistent without calibration. The ADC seems to be more accurate at higher voltage as expected. [/quote]

If I compare the measured values between measurement of the 23mV Voltage and the 500mV Voltage,
there is improvement, but not much:

0x10000024: FFFF0105 (GAIN=1, OFFSET=5) - device

[quote userid="73165" url="~/f/nordic-q-a/99981/correct-calibration-of-the-nrf51-adc-to-correct-offset-and-gain-error/427538"]Have you tested this on a nRF51 development kit as well, does it reproduce the same results? [/quote]

No I have not tested on the dev-kit until now, do you want me to do this?
If yes, I will need to re-write software for the dev-kit.

Thanks,
best regards
Philip

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

Jared — Thu, 25 May 2023 13:26:30 GMT

Hi,

[quote user="PhilipSTT"]

If I am measuring a higher voltage (in system, WITH NO bench top supply):

OK, if I try to measure a Voltage at ~500mV I get the following results:
(Taken with the 0x10000024: FFFF0105 (GAIN=1, OFFSET=5) - device)

FLUKE TRMS = ~485mV

Measurement A) = 492mV (DIFF.=7mV)
Measurement B) = 495mV (DIFF.=10mV)
Measurement C) = 494mV (DIFF.=9mV)
Measurement D) = 492mV (DIFF.=7mV)

[/quote]

I think it's difficult to evaluate these results as the FLUKE itself has some inaccuracy and it's difficult to tell what the actual voltage is. But it's clear that the calibration seems to shift the measurement from the average between the A, B, C, D.

[quote user="PhilipSTT"]If I am measuring a higher voltage (in system, WITH bench top supply):

OK, if I try to measure a Voltage at 500mV by bench top supply I get the following results:
(Taken with the 0x10000024: FFFF0105 (GAIN=1, OFFSET=5) - device)

FLUKE TRMS = ~503mV

Measurement A) = 510mV (DIFF.=7mV)
Measurement B) = 512mV (DIFF.=9mV)
Measurement C) = 513mV (DIFF.=10mV)
Measurement D) = 511mV (DIFF.=7mV)

If I do this measurement with the 0x10000024: FFFF0001 ((GAIN=0, OFFSET=1) - device:

FLUKE TRMS = ~503mV

Measurement A) = 505mV (DIFF.=2mV)
Measurement B) = 506mV (DIFF.=3mV)
Measurement C) = 506mV (DIFF.=3mV)
Measurement D) = 505mV (DIFF.=2mV)[/quote]

This is much better, and I agree that it shows that the results without the calibration is better in this case and more consistent without calibration. The ADC seems to be more accurate at higher voltage as expected.

Very good work :)

Have you tested this on a nRF51 development kit as well, does it reproduce the same results?

regards

Jared

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

PhilipSTT — Wed, 24 May 2023 14:50:28 GMT

Hi Jared,

thats good at least 🙂

If I am measuring a higher voltage (in system, WITH NO bench top supply):

OK, if I try to measure a Voltage at ~500mV I get the following results:
(Taken with the 0x10000024: FFFF0105 (GAIN=1, OFFSET=5) - device)

FLUKE TRMS = ~485mV

Measurement A) = 492mV (DIFF.=7mV)
Measurement B) = 495mV (DIFF.=10mV)
Measurement C) = 494mV (DIFF.=9mV)
Measurement D) = 492mV (DIFF.=7mV)

Error seems to get a little less, but still, with error calibration in code the measurements has the most error.

If I do this measurement with the 0x10000024: FFFF0001 ((GAIN=0, OFFSET=1) - device:

FLUKE TRMS = ~485mV

Measurement A) = 486mV (DIFF.=1mV)
Measurement B) = 487mV (DIFF.=2mV)
Measurement C) = 487mV (DIFF.=2mV)
Measurement D) = 486mV (DIFF.=1mV)

Again, the device with less error written to 0x10000024 is the more accurate one, doesn't change
if error calibration is used in code, also it doesn't really matter if TRMS measurement is done
or not (software-wise).

If I am measuring a higher voltage (in system, WITH bench top supply):

OK, if I try to measure a Voltage at 500mV by bench top supply I get the following results:
(Taken with the 0x10000024: FFFF0105 (GAIN=1, OFFSET=5) - device)

FLUKE TRMS = ~503mV

Measurement A) = 510mV (DIFF.=7mV)
Measurement B) = 512mV (DIFF.=9mV)
Measurement C) = 513mV (DIFF.=10mV)
Measurement D) = 511mV (DIFF.=7mV)

If I do this measurement with the 0x10000024: FFFF0001 ((GAIN=0, OFFSET=1) - device:

FLUKE TRMS = ~503mV

Measurement A) = 505mV (DIFF.=2mV)
Measurement B) = 506mV (DIFF.=3mV)
Measurement C) = 506mV (DIFF.=3mV)
Measurement D) = 505mV (DIFF.=2mV)

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

Jared — Wed, 24 May 2023 12:16:14 GMT

Hi Philip,

That looks correct,

Do you see the same if you try to measure a set voltage from a bench top supply? What if you increase the voltage that you measure, does it become more accurate after applying the calibration then?

regards

Jared

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

PhilipSTT — Tue, 23 May 2023 13:19:21 GMT

Hi,

Yes, I tried to implement TRMS measurement to get better results, to be closer to the values measured by our FLUKE device. Because I use it as reference, the closer I get with my ADC measurement the better.

Sure, here is my code:

Setting ADC INPUT 3 on P0.02

#define ADC_U_STATE_PIN        		NRF_ADC_CONFIG_INPUT_3     // AIN 3 P0.02

Defining DEFAULT CHANNEL

static nrf_drv_adc_channel_t     ADC_U_STATE             = NRF_DRV_ADC_DEFAULT_CHANNEL(ADC_U_STATE_PIN);             // Channel instance. Default configuration used

Which corresponds to this code (Set 10bit, full scale, ref_vbg)

#define NRF_DRV_ADC_DEFAULT_CHANNEL(analog_input)          \
{{{                                                       \
    .resolution = NRF_ADC_CONFIG_RES_10BIT,                \
    .input      = NRF_ADC_CONFIG_SCALING_INPUT_FULL_SCALE, \
    .reference  = NRF_ADC_CONFIG_REF_VBG,                  \
    .ain        = (analog_input)                           \
 }}, NULL}

regards,
Philip

RE: Correct calibration of the nRF51 ADC to correct offset and gain error

Jared — Tue, 23 May 2023 12:44:03 GMT

Hi,

I agree that, ideally the ADC result should be closer to the real value with calibration than without.

Did you use TRMS so that it would be comparable with the measurement from your Fluke device?

Could you share your initialization routine of the ADC?

regards

Jared