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LTC6951 POWER SUPPLY AND CML OUTPUT LEVELS

Hi,

We want to use LTC6951 as clock for JESD204B and want to interface it with ARRIA 10 FPGA and AD9690

We have two queries:- 

1) Whether we can interface SYSREF CML outputs of LTC6951 directly with regular LVDS IOs of ARRIA 10. I am bothered about maximum differential

output of 1600mV whether it can be given to ARRIA 10 or whether it will damage the SOC.

2) Is it essential to power LTC6951 with LDOs or whether the same can be done using DC to DC Converters of ENPIRION used to power the rest of the circuit

including SOC. Like AD9690 has clear instruction of using LDO whereas the none exists for LTC6951. However, we find that EVMs are consistently using 

LDOs.

Thanks.

  • The LTC6951 does have one LVDS output (OUT4).  The other 4 outputs (OUT0-3) are CML.  I am not that familiar with ARRIA 10, but if its inputs generate a 1.2V common mode voltage, you can AC couple the LTC6951 CML output to the ARRIA inputs. (figure 39 in LTC6951 datasheet).

    Regarding the power supply it is best to use ultra low noise LDOs, to achieve the lowest noise and spurious performance out of the LTC6951.  DC to DC converters (switching supplies) create spurs due to the switching architecture and many do not have that great noise performance.  The LT8625SP is one DC-DC converter that may work, as the power group really did a good job with that parts noise and spurious performance.  however, staying with the LT3042 or LT3045 LDOs is always a safe answer if performance is important.

  • Hi Chris,

    Thanks for your reply.

    What if we generate 5V and 3.3V through LT1117 from +12V as shown in DC2226A. We have to initially drop down the voltage through some high wattage resistor and then we can generate the same.

    +12V - HWR - 8V - LT1117 - 5V -> LT1117 - 3.3V. One qty each will generate both supplies and wattage of both regulators will also be in check to avoid complications of thermal designs.

    What are your views on it?

  • I think I will move this one over to the power group (I'm the clock guy).  Can you estimate how much current you will have in each supply based off the LTC6951 datasheet power supply current vs number of outputs on typical plot?

  • On the DC2226 board the LTC6951s 5V supplies are generated from the LT3042.  The LTC6951s 5V supplies (VCO, and Charge Pump supplies) are more sensitive to supply noise and spurious content than the 3.3V supplies.  The LT3042 has lower noise and PSRR than the LT1117, so that is why it was chosen for the 5V supplies.  Also, notice the large 47uf cap on the output of the LT1117, that was used to help reduce noise and spurs on the 3.3V supplies.  Power group can help with the thermal questions.  I've emailed them independently to look at this engzone post.