ST25R3911B Hardware noise issues.

[Huzaifa_Tariq01]

Hi, I am using an ST25R3911B using the ST25NFC_embedded_library in polling mode on an stm32f103CBT6. In my program, I poll my ST25 and then move on to reading values from arrays of hall sensors (4 arrays of 16 sensors each) a total of 64 sensors. The issue I am facing is that, after a number of stable and precise repetitive readings, i get a complete reading with noise in the readings. when I shutdown polling of the ST25 in my software, the noise disappears. The left sensor array is most affected by the noise. 

Note:: Also the noise is not very random, every time the shift in sensor readings for each sensor is almost same as show in error log.

I have attached an image of the serial debug terminal highlighting the noise.

[Travis Palmer]

Hello Huzaifa_Tariq01,

If I understood your post correctly, you assume that the NFC reader is disturbing the hall sensors.

Hall sensor are based on magnetic sensing. In your setup, can they be disturbed by the magnetic field sent by the ST25R device?

You could check by taking another ST25R3911B reader (e.g. ST25R3911B-Disco board), emit a continuous magnetic field and bring it close to the hall sensors while reading them out.

BR Travis

[Huzaifa_Tariq01]

Our team has tested the interference of RF signals with the Hall sensor arrays, there is no effect on the sensor readings. However, when ST25R is turned on it generates lots of noise in system's ground (tested using oscilloscope). The noise in the sensor arrays is some sort of grounding issue and voltage irregularities being caused by the ST25.

[AScha.3]

So the problem comes from wrong/bad ground in your hardware setup.

Try to separate the currents flowing in ground (and power) connections.

Maybe at first, take a piece of paper and draw the connections, where current is going - and to avoid the mix of the currents from ST25R with the Hall sensors ground+power .

[Travis Palmer]

Hello Huzaifa_Tariq01,

I would then recommend to split the power supply for the hall sensors and the NFC section (maybe add an additional LDO) to increase PSRR and split the grounds.

As Ascha.3 highlighted, a current flowing on a common ground track can cause a common voltage. It means although no current is flowing on the hall sensors ground path, the NFC current can lift up your hall sensors ground potential, which could cause interference (noise) on your hall measurement. You can also try to reduce the power consumption of the ST25R3911B.

One thing that is strange to me is that the interference is not permanently there. I assume this is due to the polling of the ST25R3911B and the read-out of the hall sensors only sometimes collide. You can also try to time multiplex between NFC operation and hall sensor read-out.

Can you share pictures and measurements of your setup and PCB layout?

br Travis