Is the VSS Pin (pin 103) connected internally to GND_Shield?

[Stefan Schmidt]

Dear Nordic Support Team,

 In the datasheet of the nrf9160 (nRF9160_PS_v2.2.pdf) the VSS pin appears in the section 6.4.5.1 GPIO Electrical Specification, but I cannot find the information if it is internally connected to GND_Shield.

Best regards

Stefan

[Øyvind]

Hello Stefan, 

No sure what you are referring to. The GND_Shield pins of the nRF9160 are connected to Ground, similarly the VSS is connected to Ground. This is described in chapter 10 Hardware and layout. 

Kind regards,
Øyvind

[Stefan Schmidt]

Hi Øyvind,

thanks for your reply. I know that they have to be connected them to ground, my question is: Are they connected in the nrf9160?

I tried to read VCC_GPIO on a custom board and was wondering why the readings are so far away from the 1.778 V I was measuring with an external Multimeter. The readings are outside the 4% DC error, and always lower than 1.779 V. I increased the aquisition time to 40 µs, but this did not solve the issue.

When the modem was active the VCC_GPIO read with the ADC was even lower, so I thought that ground bouncing might be the issue and that it would be better to have the VSS pin routed to the star ground point separately from the GND_Shield pins (which I suspected to be the ground pins for the modem).

But now I have set up this sample zephyr/samples/drivers/adc on a nRF9160-dk and this is what I read for the 1.8 V VCC_GPIO:

- adc@e000, channel 1: 7621 = 1674 mV

I use this devicetree setup for this channel:

    channel@1 {
        reg = <1>;
        zephyr,gain = "ADC_GAIN_1_6";
        zephyr,reference = "ADC_REF_INTERNAL";
        zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
        zephyr,input-positive = <NRF_SAADC_VDD>;
        zephyr,resolution = <14>;
        zephyr,oversampling = <8>;
    };

Do you have an idea why the reading is so wrong?

[Øyvind]

Hi Stefan, my apologies for the late reply. The R&D team have been looking into this, and they have the following questions in order to understand the situation:

1. When you use the DK, are you using single ended mode? (I have no idea how to read the zephyr config below, that’s far above the SAADC config)

2. If you’re using single ended, it would be interesting to switch to differential, and connect the negative side to the exact ground that you use for the multimeeter where you measure the VDD voltage. Ground bound will affect the measurement, but 80 mV is signficiant ground bounce.

Kind regards,
Øyvind

[Stefan Schmidt]

Hi Øyvind,

 now I have to apologize, I have been on vacation last week :-)

Thank you for your reply. I have been using single ended mode, and the idea to use differential mode and connect the negative side to the 'star' ground is actually a good one! I will try this. 

Best regard

Stefan

[Øyvind]

Stefan Schmidt said:

now I have to apologize, I have been on vacation last week :-)

No worries! Hope you had a good vacation and able to charge the batteries ;)

Stefan Schmidt said:

Thank you for your reply. I have been using single ended mode, and the idea to use differential mode and connect the negative side to the 'star' ground is actually a good one! I will try this. 

Yes, please test and let me know how that works for you.

Kind regards,

Øyvind

[Stefan Schmidt]

Hi Øyvind,

 I have setup the tiny project on the 9160DK board with one channel single ended, the other channel in differential mode with the negative input connected to ground externally.

Now, that all leaves me a bit unsatisfied. I measure 1.817V with a DMM on VDD_GPIO, but both channels give me the same reading of about 1.696mV:
ADC reading[0]:
- adc@e000, channel 0: 1929 = 1695 mV
- adc@e000, channel 1: 965 = 1696 mV
ADC reading[1]:
- adc@e000, channel 0: 1929 = 1695 mV
- adc@e000, channel 1: 964 = 1694 mV
ADC reading[2]:
- adc@e000, channel 0: 1930 = 1696 mV
- adc@e000, channel 1: 964 = 1694 mV
ADC reading[3]:
- adc@e000, channel 0: 1929 = 1695 mV
- adc@e000, channel 1: 964 = 1694 mV
ADC reading[4]:
- adc@e000, channel 0: 1929 = 1695 mV
- adc@e000, channel 1: 964 = 1694 mV
ADC reading[5]:
- adc@e000, channel 0: 1930 = 1696 mV
- adc@e000, channel 1: 965 = 1696 mV
ADC reading[6]:
- adc@e000, channel 0: 1930 = 1696 mV
- adc@e000, channel 1: 965 = 1696 mV

I 2350.adc.zip

I have attached the zipped zephyr/samples/drivers/adc folder, you can find my nrf9160dk_nrf9160_ns.overlay in boards.

If you find some time, could you please have a look a the program and give me some hints how I could get the reading closer to the 1.8V that I measure with my DMM?

Best regards

Stefan

[Øyvind]

Hi Stefan thanks for sharing. No updates yet. 

Kind regards,
Øyvind