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Frequently Asked Question

What causes a Quadrature Digital Upconverter (AD9856, AD9857) to go into a CIC overflow condition?

The most likely causes of a CIC overflow condition are as follows:

  1. There is excessive jitter on the REF CLK (This can be exacerbated when you are using the internal REF CLK multiplier).
  2. The REF CLOCK multiplier PLL becomes unlocked while TX enable is high.
  3. Profiles were changed (by pins PS0, PS1) while TX enable was high.

Both the AD9856 and the AD9857 can exhibit a CIC overflow condition if any of the above occur. The AD9857 has a CIC overflow pin (pin 69); if this pin begins to toggle (anything but staying low), a CIC overflow has occurred. The AD9857 also allows the lock status of the PLL to be monitored (pin 68). If the PLL is becoming unlocked during TX operation, then there is probably excessive jitter on the external reference signal.