Hello Ramon,
Every component on the PCB (and the PCB itself) will change its behavior when being exposed to temperature. This effects the PCB antenna, EMI inductors as well as the CR95 itself and will therefore change the voltage across the NFC antenna and measured current.
The above mentioned behavior of your experiment could be linked to heating of the EMI inductors. For example, if the field has been turned on (e.g. due to polling for a tag for several 100ms) before the calcRef is executed. This will heat up the inductors (depended on their current rating) and produce a calcRef value which is too high - or equivalent to higher ambient temperature. Your higher level and lower level might be shifted. As a result the device would wake up faster when being cooled down, but slower when being heated up. It would be interesting to know what the current consumption during continuous Tx of your board is and what type of inductors you are currently using. Maybe you can measure the temperature using an IR camera? It would also be interesting if you could share the refCal values for 25°C, hot and cold.
In general less antenna windings will cause a higher current flowing through the antenna. Be aware that any change which might be favorable for the wake-up performance might effect other modes of the IC - which could for example reduce the sensitivity and therefore read range.
Increasing the offset between lower and upper limit could be one other solution. But it will reduce for sure the detection range.
Looking at our ST25R3916 or ST25R3911B the principle is quite similar to the CR95HF. There it is a common procedure to obtain the reference value every time before going to enter wake-up mode. This is a common practice since these devices are even more sensitive and therefore more sensitive to such environmental effects.
Criteria for good EMI inductors are:
- Q-Factor => crucial for performance ( I guess not so important for your application)
- low DC R => very interesting for you - less losses and heating
- High saturation current => same as above
- temperature stability
I am not sure, if changing the EMI cut-off will influence the temperature behavior. More likely playing the inductance value (while staying at the same cut-off) could have a positive effect => lower inductance value = lower DC resistance. (for example if it is related to inductor heating).
The ST25R95 and CR95HF should behave the same in this regard.
BR Travis
