How can we find accurate voltage in ADC channel in nRF9160?

[jaydip kavaiya]

Hi All

We are using nRF9160 DK, and we are working on ADC channel. we use This example of ADC.  I am using single-ended with no over-sampling with a gain of 1/6, reference 0.6.

10 bit ADC resolution. we want to find voltage given by power supply to nRF9160 like 1v, 1.5v, 2v but getting value is less than the input value

like I share some reading

Applied Voltage (mV)	Raw Value(count)	Displaying Voltage(mV)
1003	260	914
1500	400	1404
1999	540	1900
3001	612	2153

[jbrzozoski]

Did you happen to switch your dev kit up to 3V GPIO?  There was an errata with the older demo silicon on a lot of the dev kits:

infocenter.nordicsemi.com/.../anomaly_160_16.html

[jaydip kavaiya]

Thanks for the reply 

I do not want to switch my kit to 3V GPIO. I read this link like if VDD_GPIO higher than 2.1 V. Not able to read out correct value from SAADC output. But my only question is 

I provide supply voltage to nRF9160 DK is below then 2.1V but still, I am not getting correct value is there any suggestion for me

I used the ADC example with no Changes.

[Sigurd]

Hi jaydip,

What version of the DK do you have(should be printed on white sticker) ? is it v0.8.5?

What is the position of the VDD_IO? Is it set to 1V8 or 3V ?

[jaydip kavaiya]

Hi Sigurd

It is v0.8.5.

and VDD_IO is set to 1V8.

Thanks In Advance

[Sigurd]

Hi,

The absolute maximum voltage you can supply to a GPIO pin is VDD_GPIO (+ 0.3). VDD_GPIO is on the DK the same as VDD_IO. See the Absolute maximum ratings chapter in the PS. Outside this value you will not get valid results, and you risk damaging the chip.

jaydip kavaiya said:

we want to find voltage given by power supply to nRF9160

Instead of using SAADC for this, you should instead use the AT command %XVBAT. See this link

Using the %XVBAT command, measuring battery voltage up to 5.5V is supported.

[jaydip kavaiya]

Hi Sigurd

Thank for your Valuable reply

We are testing ADC value on Link monitor like this.

We are giving voltage to nRF9160 DK via This type of Power supply in Pin no. P0.13.

and the voltage range is like 

supply voltage (mV)                     Showing in Link monitor (mV)         Difference(mV)  

1503                                             1393 to 1412                                   95-110

1202                                             1097 to 1105                                   90-105

We are using This Example with No changes

How can I get the exact value?

Thanks in Advance

Jaydip

[Sigurd]

Have you verified that the output from the power-supply is accurate? E.g. with a external multimeter?

I’m seeing ~30 mV accuracy on a v0.8.5 nRF91-DK with 1.8V input signal.

[jaydip kavaiya]

Hi Sigurd 

I checked with Multimeter from both sides like where are give supply and from the board, it's accurate e.g.

like if I provide 1202 so on multimeter showing exactly 1202 and board pins also same but the result is

not showing accurate like its 100 or 95 mV is less.

Thanks & Regards

Jaydip

[Sigurd]

There is some chip to chip variation, so on some DK’s you can have ~100mV accuracy for single-ended input measuring at these voltages. On other DK’s you might get a better result because of the chip to chip variation. Looking at the specification for the SAADC at this link, the error for the 1/6 gain is from -3 to 3%. You also have some temperature drift (typ 0.02 %/C), and maybe around 2-4 LSB single-ended offset error.