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ST95HF crystal characteristics

Question asked by Tobias on Sep 26, 2016
Latest reply on Sep 27, 2016 by Peter Boringer

we are using ST95 HF NFC transceiver with XRCGB27M120F3M00R0 27.12 MHz crystal. So far, we manuufactured about 30 modules and all works well. However, we are planning a bit larger series in a close future (10k+) so we would like to be sure everything will work fine in whole operating range. Currently, we have concerns regarding a crystal specification.

In datasheet (p 59) it is stated that with a crystal of ESR equal to 30 ohms, rated load capacitance should be 6pF, and also that used load capacitors should be in 10 to 20 pF range.

If a crystal ESR is 5 times larger (150 ohms), does that mean that rated load capacitance should also be larger (and how much) in order to keep compatibility with embedded oscillator?

We are using mentioned crystal that has 150 ohms ESR and load capacitance of 6pF. We are expecting about 5 pF of stray capacitance (including pin and PCB stray capacitance), so we are using 6.8pF load capacitors. Could this also be a problem, since they are not in 10 to 20 pF range?

If datasheet had specified oscillator transconductance, that could be also helpful...