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LTC2175-14 duty cycle stabilizer

Category: Hardware
Product Number: LTC2175-14


I am using the DC1525A-A evaluation board (LTC2175-14) with a Xilinx SP605 FPGA board for direct digital phase noise measurement using the cross-correlation averaging method:

I feed a a DUT oscillator into two channels via a power splitter, and two independent high-quality reference oscillators into the other channels.  I then measure phase difference between DUT and REFs, thereby cancelling clock jitter, and perform FFT, cross-correlation and long term averaging.

I originally used the 16-bit serialization mode but recently switched to 14-bit serialization in order to reduce the high-speed data rate.

My setup detects a very small systematic measurement bias, which until recently produced anti-correlation between the two independent measurements.  But since switching to 14 bit mode, the bias has reduced and reversed polarity!

I wonder if the clock duty cycle stabilizer could be the cause?  May we know more about how it works?  Is it a digital scheme that operates at the DCO frequency?  My DCO rate has changed by a factor of 7/8.

I drive ENC with a high-purity sine wave and have modified the DC1525A-A board as per the ADC datasheet to accept sine wave drive.

I have searched this forum for information about DCS and found only two statements relating to other parts, one saying it adds jitter, and the other saying a small pseudo-random charge put on the sampling capacitor is not cancelled.  Any pseudo-random process that is common (in phase or anti-phase) between two channels will be detected by cross-correlation averaging.

Thanks, Andrew.

  • Hi,

    We'll look into this. We'll contact the product owner and get back to you.

    Best regards, 


  • Hi Aholme,

    To give you a more insight on this one, the serialization and framing of 16-bit and 14-bit are different. For 16-bit, 1 sample is converted in one frame while for 14-bit, 2 samples are converted in 1 frame.

    Please take note that I am not the product owner of this one, but out-of-curiosity's sake, hope this information helps. 



  • Hello Xavier and Peevee,

    I have read the datasheet in detail many times and I am very familiar with the published differences between the modes.  I was hoping for unpublished insights.

    The SP605 + DC1525A-A combination have been very valuable tools for the hobbyist community and I was deeply saddened and disappointed to learn this week that AD have obsoleted the DC1525A-A and it is no longer available through distribution.  If any remaining inventory is available, I would be grateful if someone from AD could contact me offline.  Up to now, users of my phase noise measurement system have sourced their own hardware.  The software and firmware are free to download from my website.  But I could hold a small stock of ADC boards for new users in future.