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DAC3484 PLL ?

HENRY JUNG
Expert 2030 points
Other Parts Discussed in Thread: DAC3484, DAC34SH84

Dears.

We are testing with DAC3484

We would like to inquiry about DAC3484 of PLL.

VCO RANGE is 3.3GHz ~ 4.0GHz by Datasheet.

But as we input 4000MHz in below formula, Coarse tuning bits is out of setting range.

So we have no choice, only set Coarse tuning bits to 63 can read pll_lfvolt(2:0) to‘0’.

Please help to check how can use PLL with 4000MHz VCO Frequency.

 ** Please refer to the below PLL parameter.

DACCLK is 200MHz.
Target-Frequency is 800MHz.
Interpolation is x4.

fVCO 

4000 MHz

PFD

40 MHz

N

5

M

20

P

5

pll_vco

63

P*M(=overall divide ratio)

100 (single charge pump)

 

 

 

 

 

 

 

  • 0
    TI__Guru* 78614 points
    Henry,

    We cannot recommend such operation. The PLL due to the VCO range will definitely loose lock over the entire temperature range.

    800MHz happens to be an odd frequency in which requires the VCO of 3200 or 4000, which is not supported well.

    You may need to look into the DAC34SH84, which has a recentered VCO near 3000Mhz. With P =4 and VCO of 3200MHz you may be able to reach 800MHz of DACCLK rate. See figure 10 of the DAC34SH84 for recommended VCO programming for detail.

    -Kang
  • 0 in reply to Kang Hsia
    Expert 2030 points
    kang.

    VCO RANGE is 3.3GHz ~ 4.0GHz as datasheet, so we use it with DAC3484, but it cannot support 4Ghz VCO.
    We are in a mess about that. We cannot understand why it could not support 4Ghz according to temperature changing.
    We would like to know the details of why cannot support VCO 4Ghz.
    And please help to check if there is any way can use VCO 4Ghz.
    If not, we can only use DAC34SH84? DAC34SH84 & DAC3484 are Pin to Pin?
    Please help to check.
    Thank you.
  • 0 in reply to HENRY JUNG
    TI__Guru* 78614 points

    Henry

    DAC34SH84 is pin to pin to DAC3484IZAY package (BGA). It requires slightly elevated supplies for the 1.2V rails.

    The 4.0GHz is right at the edge of the VCO range. The VCO is LC tank based with varactor (variable/programmable capacitor banks programmed by the VCO coarse tune bits). At the edge of the VCO there is not enough varactor banks to give further range. therefore, at 4GHz, the PLL/VCO can still be functional, but may go overrange perhaps at extreme cold temp or hot temperature

    You may always adjust the coarse tune code for the VCO across the temperature and resync the DAC accordingly. The resync procedure is below