nRF52832 ADC differential mode configuration

RE: nRF52832 ADC differential mode configuration

Kenneth — Tue, 12 Feb 2019 09:08:49 GMT

You have several options here, you may use VDD as reference and sample the two signals individually and calculate the difference, or sample the two signal as one differential signal. If you don't want to use VDD as reference, then you can use internal 0.6V as reference instead, but then you should enable 1/6 gain setting to make sure the measurement are below internal <0.6V reference.

RE: nRF52832 ADC differential mode configuration

kmalmiv16 — Mon, 11 Feb 2019 21:53:18 GMT

VDD is 3,3V, regulated.

Rgds,

-KM-

RE: nRF52832 ADC differential mode configuration

Kenneth — Mon, 11 Feb 2019 20:59:35 GMT

I think I understand, what voltage is VDD here?

Best regards,

Kenneth

RE: nRF52832 ADC differential mode configuration

kmalmiv16 — Mon, 11 Feb 2019 20:29:21 GMT

Thanks Kenneth, this was good addition.

I will check the acquisition time!

We cannot use 3 channels, as we only have two signals.

And actually it is not pure differential signal, it is difference between two "floating" signals we are measuring with ADC.

This means that other is "fixed" 1.65V and other varies between 0,5V...2,8V. These two signals we should sample and report the difference.

Do you recommend to do it in sequence of two SE-samples?

If we do it in differential mode, does ADC internally report the difference of two signals as a result?

Rgds,

-KM-

RE: nRF52832 ADC differential mode configuration

Kenneth — Mon, 11 Feb 2019 12:58:36 GMT

Hi,

I suggest to start with the saadc example in the SDK:
https://www.nordicsemi.com/DocLib/Content/SDK_Doc/nRF5_SDK/v15-2-0/nrf_dev_saadc_example

To change between single ended and differential you only need to use a difference macro when you init the channel(s):
NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(PIN_P) vs.
NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_DIFFERENTIAL(PIN_P, PIN_N).

In your case I suggest to setup 3 channels, the third channel is the reference voltage (this may also be differential). There are a some advantages by also sampling the reference voltage, you will reduce gain and offset errors in the saadc by directly comparing digital value of the analog input signal(s) vs. analog input reference voltage measurement.

You can enable all 3 channel with oversampling, burst, and scan. Then when you start sampling you will get an event when all 3 measurements are done.

I recommend to use 10us acquisition time or more due to these Errata's:
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.Rev2.errata/dita/errata/nRF52832/Rev2/latest/anomaly_832_74.html?cp=2_1_1_0_1_14
http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.Rev2.errata/dita/errata/nRF52832/Rev2/latest/anomaly_832_150.html?cp=2_1_1_0_1_40

Best regards,
Kenneth