In some application notes, there is a recommendation to use an external resistor in series with the OSC_OUT pin to reduce the power dissipated by the crystal, if necessary to do so. A good example is from AN2867 STM32 oscillator design guide called "R_Ext".
Note there is never a resistor on the OSC_IN (EXTAL) leg in any application note and thus I never considered it for any transconductance calculation (Note 1).
So I've been a little bit thrown out by the S32K datasheet which explicitly shows an extra (internal) 280R resistor on the OSC_IN (the S32K calls the pin EXTAL) pin (p27, Figure 8):
So, my questions are:
- Does this "resistor" have any effect on the transconductance (gain margin) of the circuit design? EDIT - Supplementary question: If it does affect the transconductance, how is this calculated?
- Is it likely to introduce any other effects that needs to get taken into account in the design?
- Would there ever be a reason to deliberately design in a resistor connected the inverter input?
Unfortunately, the 280R is 2-3 times larger compared to the ESR of crystals in the 16MHz range (in the order of 100R, e.g. this or this), so if it does indeed affect the transconductance calculation, the effect is non-trivial to the point where the circuit may refuse to oscillate.
The transconductance calculation I am referring to is the modified form that takes R_Ext into account as explained in the STM32 document referenced earlier:
Part of the question is whether the resistor on OSC_IN needs to be included anywhere in this calculation.