Hi Team,
I found the following forum about crystal electrical model for AFE4400(AFE4490)
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Question:
What is the negative resistance of the XIN and XOUT pins? What is the crystal electrical model that will work with AFE4490? What should the crystal ESR be?
Answer:
For the oscillator to start up and sustain oscillation requires the negative resistance achievable inside the circuit to be at least about 3 times the ESR of the crystal.
The negative resistance is given by:
R =-1/{2*w* Csh*[1+Csh*(C2+C1)/(C2*C1)]}
where w is the frequency of oscillation in rad/s, Csh is the shunt cap of the crystal and C1,C2 are the caps to ground from the XIN,XOUT pins.
With Csh=8pF, C1=C2=15 pF, at a frequency of 8 MHz we get:
R= -600 Ohm
So we want the crystal ESR to be limited to less than about 200 Ohm, lower the better.
One way to increase the magnitude of negative resistance achievable is to increase the values of C1 and C2. However there is a limit on this based on the internal circuit of the AFE. So we recommend using a value of C1, C2 of about 15 pF.
https://e2e.ti.com/support/applications/high_reliability/f/30/t/306018
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In general, I believe oscillation margin decrease if load capacitance increase.
Could you tell me the background of the following equation?
>R =-1/{2*w* Csh*[1+Csh*(C2+C1)/(C2*C1)]}
Best Regards,
Yaita
