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DAC37J82: DAC37J82 sysref circuit hardware configuration

Brooke
Prodigy 120 points
Part Number: DAC37J82
Other Parts Discussed in Thread: , LMK04828

Tool/software:

Dear TI support team,

We are trying to use the DAC37J82 in our project, and we got the DAC37J82EVM on hand.

In the datasheet of DAC37J82, the SYSREFN/P pin required LVPECL electrical standard.

But with demo configuration example, we find here you use LCPECL output from LMK04828.

I check your demo schematic below as well, and this is real LCPECL setting hardware configuration.

Could you please let me know here why you use LCPECL interface for the SYSREFN/P of DAC37J82?

In datasheet of DAC, that's no defined this LCPECL can support SYSREFN/P.

Thanks for your time.

Best Regards,

Brooke

  • 0
    TI__Expert 4480 points

    Hi Brooke,

    This is probably a typo in the LMK04828 GUI as I am not aware of any standard called LCPECL.  Let me verify this with them.

    Regards,

    Geoff

  • 0 in reply to Geoff Roth
    TI__Mastermind 22310 points

    Brooke,

    The name of the standard does not matter as the common mode and swing requirements are met for the DAC. This DAC wants to see a 800mVpp signal centered around 500mVdc. With that considered:

    The LMKs LCPECL is a lower common mode voltage than the LVPECL 2000 output format. The swing is considered the same around 950mVpp for both but the high and low voltages are different. If we were AC coupling, then you would use LVPECL2000mV and be done, but since this sysref is DC coupled, the common mode must be met. So the LCPECL output mode provides a common mode of (1.57+0.62)/2=1.095Vdc. The LVPECL2000 has common mode of (2.21+1.25)/2=1.73Vdc.

    On the DAC EVM, the termination structure is simply LVPECL/LCPECL 240R pulldown, then after this instead of typical AC coupling caps, it passes through a voltage divider, which cuts swing and Vcm in half for each differential leg. This makes the common mode in the LCPECL mode at 0.547mVdc while if you went to LVPECL2000 then common mode would be near 0.85V which is too high.

    The 575mVdc is not a perfect match for 500mV common mode, but this can be accounted for in the voltage divider to attenuate slightly more as long as component selection allows it. I wasn’t around when this board decision was made, but I imagine it was to get compliant enough to support DC coupled sysref input, and maintain a high swing. If you attenuate to have Vcm perfectly match 500mVdc then you also attenuate the differential swing. Note that if you do change the voltage divider resistors, that the sum of the voltage divider resistors MUST match the value of the first LCPECL/LVPECL pulldown, in the case above 200R, this way each leg is terminated to GND evenly through the attenuator. The second note is that the equivalent resistance to GND should be within 100 to 240 ohm for the LMK output termination.

    Chase