AD724

Document created by analog-archivist Employee on Feb 23, 2016
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Currently we are applying the AD724   RGB to NTSC/PAL encoder.
The schematic is from the data sheet page 11.
Now our production is faced with oscillator start up problems.
About 20% of the IC's have oscillator start up problems.

The Xtal we aplly is from manufacturer TXC
    4.433619MHz
    fundamental
    HC-49/U
    shunt capacitance 7pF

The board capacitance itself is low
On the PCB is a capacitance of 10pF parallel to the Xtal.
Changing this capacitance to 20pF does not matter for "good" IC's
The "bad" IC's sometimes start up with smaller than 10pF.
On the PCB the Xtal is quit close to the IC.
The IC's with start up problems have lot code     0019 and 9952.

 

The oscillator circuit on the AD724 is designed to oscillate only over a very
narrow frequency range centred on 4.433619MHz.If the tuned circuit which it is
connected to is not resonant at or very near this frequency then of coarse the
circuit will not oscillate.

The AD724 requires a parallel resonant crystal. A parallel resonant crystal
requires a shunt capacitor in order to tune it to the right frequency and this
value will be specified by the manufacturer. The shunt capacitance is not there
to "kick start" the crystal but instead tunes the circuit to the required
frequency. The recommended value of capacitiance is 7pF. If you have a larger
value of capacitance then the circuit will not be tuned to the correct
frequency and the oscillator circuit may not operate. Of course some parts will
oscillate but some will not. On the parts that do oscillate with a 20pF shunt,
try measuring the actual frequency of oscillation.

As a solution, I would suggest that you need tighter control over the shunt
capacitance. Ensure the the crystal is mounted close the AD724 and remember to
account for parasitic capacitnaces in your PCB.

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