Getting different ADC readings when using JLink debugging than when not

[i_am_trying]

I have built a PCB that includes the nRF51822. I have AIN7 reading an input voltage that I measure on my DMM at 269mV.

When I measure the voltage through the nRF51822's ADC, I get 261 mV when the code is run through Eclipse debugging with the nRF51 DK's debug out hooked up to the SWD lines of the nRF51822 on my board.

When I run with just my board hooked up to a 3V power source, I get 213mV reading from the ADC.

Why would the readings vary by 48mV? (with the reading when debugging using the SEGGER SWD channels being 48mV more?).

The app I am running uses the SoftDevice S110 stack. Here is the ADC Code:

    NRF_ADC->CONFIG	= (ADC_CONFIG_EXTREFSEL_None << ADC_CONFIG_EXTREFSEL_Pos) /* Not using an external reference for AREF */
											| (ADC_CONFIG_PSEL_AnalogInput7 << ADC_CONFIG_PSEL_Pos) /* Sets which AIN (0-7) to sample from */
											| (ADC_CONFIG_REFSEL_VBG << ADC_CONFIG_REFSEL_Pos) /* use the internal 1.2V bandgap voltage as reference */
											| (ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling << ADC_CONFIG_INPSEL_Pos) /* use 1/3 prescaling */
											| (ADC_CONFIG_RES_10bit << ADC_CONFIG_RES_Pos);	/* use 10 bit resolution when sampling */
/*!
 * \brief *->enable the ADC by setting the NRF_ADC->ENABLE register
 */
NRF_ADC->ENABLE = ADC_ENABLE_ENABLE_Enabled;
/*!
 * \brief *->an ADC sample starts when NRF_ADC->TASKS_START is set to 1
 */
NRF_ADC->TASKS_START = 1;
/*!
 * \brief *->as noted in the nRF51_Series_Reference_manual v3.0.pdf: "During sampling the ADC will enter a busy state.  The ADC busy/ready state can be monitored
 * via the BUSY register."
 * \note The "White paper content - nrf51 ADC.pdf" states: ..."multiple samples are made and the ADC output value is the mean value from the sample pool...the sample pool is created
 * during 20µS period for 8 bit sample, 36µS for 9 bit sampling, and 68µS for 10 bit sampling.  I originally was going to take multiple samples and average - but heck, looks like the
 * smart folks at Nordic took care of this! :-)
 */
while (NRF_ADC->BUSY){
}
/*!
 * \brief *->the results are ready to be copied from the NRF_ADC->RESULT register.
 */
int16_t adc_result = NRF_ADC->RESULT;
/**
 * \brief *->while 31.1.6 of the nRF51_Series_Reference_manual v3.0.pdf poings out the ADC supports one-shot operation, the code seems to still have to tell the ADC to stop
 * using NRF_ADC->TASKS_STOP = 1;
 */
NRF_ADC->TASKS_STOP = 1;
/*!

[Stefan Birnir Sverrisson]

Hi

I dont think I have seen this issue before. Some thoughts:

• Is the result different if you use another analog input pin, e.g. #5 ?, just thinking that there is possibly another adjacent pin that has some activity that causes disturbance.
• Do you have LPCOMP enabled. That is known to cause some issues.
• What are you measuring on the analog input pin? Does the external circuit have high impedance?

[i_am_trying]

re: LPCOMP. I truly don't know. The code I show above is how I understand getting a reading of the ADC should work. I have read that if the softdevice is there, LPCOMP should be disabled. But not sure what to add to the code above?

I am using the same power source that is powering the nRF51822 to create a voltage divider. The circuit should not have a high impedance. ?

[i_am_trying]

Update: I checked the AC side worked as expected. E.g.: on a AIN I expected to see 1.46V going in. This was true in all set ups.

I then disabled LPCOMP. I also changed the macro for the clock - I do not have an external crystal on my board. Neither of these changes made a difference in getting 1.2V when NOT going through Eclipse debugger.

So - since I observe this behavior only when the SWD lines are being used for debuging (i.e.: nrf51 DK is providing debug out), i assume there is something different when Eclipse/debug reads/writes to the SWD lines? Does this make sense? Any thoughts on why?

[Stefan Birnir Sverrisson]

LPCOMP: ok, that is not the issue then. The LPCOMP is disabled by default, so if you have not manually enabled it, it should not be enabled. So that is clearly not the issue.

The circuit connected to the analog input pin should not have high impedance, optimally lower than 1 kohm, even though 5 kohms should be without causing considerable error on the ADC. If you have higher impedance, there are workarounds, e.g. with connecting a capacitor between the AIN pin and ground, as described on this thread and this thread.

[i_am_trying]

Thank you. The trace going into the AIN is from an out pin of an op amp. So the impedance should not be an issue. Any other thoughts?