About the problems of output exception after knocking on AD9957. How CCI filter works in AD9957.

Is CCI filter in AD9957 similar withe the general form of a cascade? Does it only use the adder and negated device? Then what is the position of overflow when the overflow indication of CCI is high at the pin 20 , and after overflow what  is CCI filter output signal?

1. Problem Description

  DA converter used is AD9957. During normal operation, the spectrum broaden to error after DA is rapped by insulation. The pin 20 goes high, indicating CCI filter overflow.

2.Test

    1)Abnormal, continuous power output spectrum can not be restored. Re-power can be restored

    2)After checking the welding is not a problem

    3)The data and clock from FPGA is OK.

3.Configuration Instructions

    Mode is QDUC without BFI; output frequency is 46.5MHz; Bandwith is 20MHz; SYSCLK is 248MHz; PDCLK is 62MHz; Difference factor R=2; No use PLL; No use SYNC mode.

4. Symptoms Description

  1)Normal output with frequency 46.5MHz and span of Spectrum Analyzer is 50MHz


2)Abnormal output with frequency 46.5MHz and span of Spectrum Analyzer is 50MHz

  3)Abnormal output with span of Spectrum Analyzer is 50MHz



  • 0
    •  Analog Employees 
    on Apr 29, 2015 5:49 PM

    Hi,

    We are currently working on your inquiry.

    Best Regards

    Louijie

  • 0
    •  Analog Employees 
    on Apr 30, 2015 10:18 PM

    A CCI overflow is not self correcting. That is, once the overflow condition occurs it will remain until you perform a "CCI clear" via CFR1[21], which resets the CCI filter.

    Although a CCI overflow is unusual, it can be caused by an abrupt change of the system clock phase or frequency, excessive jitter on the system clock, or by programming a different "R" value on the fly. Otherwise, it should be difficult to produce a CCI overflow.

    You mentioned the problem occurs when the device is "wrapped with insulation". If thermal insulation, then it is possible that the device is overheating. Overheating could cause a CCI overflow if the temperature increases to the point of making the system clock erratic (i.e., high jitter).