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Clock specifications for AFE4400 / AFE4490

Praveen Aroul
Mastermind 49926 points
Other Parts Discussed in Thread: AFE4490

Question: Is it possible to run the AFE4400 on a buffered clock input rather than an external XTAL?

Answer: Yes it is possible to connect a buffered clock input on the Xin pin of AFE4400. This requires that XTALDIS (bit 9) CONTROL2 Register (Reg 0x23h) is set to HIGH.

Question: Why is 8 MHz the only supported frequency?

Answer: The AFE4400 has been characterized for 8MHz frequency only. Running at higher clock frequency will likely affect the ADC in the receive signal chain and impact the performance adversely.

Question: Does TI have any recommendation for a small size 8MHz crystal?

Answer: For a small size crystal, our recommendation is the 8MHz Piezo Ceramic resonator crystal (CSTCE8M00G55-R0) which has the following dimensions (1.3 x 3.2 x 0.7mm)

Question: What is the required accuracy of external clock for AFE4490 (frequency tolerance)?

Response: The AFE4490 can be provided with an external clock that operates at an accuracy of +/-2% around specified value of 8 MHz.

  • 0
    TI__Mastermind 49926 points

    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.

  • 0
    Expert 1940 points
    Thanks for the info.
    If I supply an external clock to the XIN pin, can the XOUT pin simply be left "open", or should it be tied high or low?
    Thanks
  • 0 in reply to Ed Hepler
    TI__Mastermind 49926 points
    With external clock on XIN pin, XOUT must be left open.