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Original question:

AFE7225: what is the pullup and pulldown value resistor is required for DAC output pins to achieve full scale swing and 20mA...

AFE7225: AFE7225: What is the expected impedance of each AFE DAC output pins

Pradeep Prabhu G
Intellectual 560 points
Part Number: AFE7225

Hi,

As per the current sync DAC requirement, We have given 71.5 ohm pullup to P5V0 and 165 ohm pull-down to ground to maintain 50ohm impedance in AFE DAC output (IOUTP/N_A_DAC and IOUTP/N_B_DAC).

In assembled board, Due to this pullup and pulldown resistor the complete 5V power rail impedance become around 50 to 55ohm.

For your information, In our design we have connected this same P5V0 power to other sections as well.

Could you please let us know is this 5V power rail impedance is expected if we connect this power to current sync DAC using 71.5Ohm pull-up resistor?

What is the expected impedance of AFE DAC output pins?

Is this 5V rail impedance is OK to power ON the boards or Is there any solution to increase 5V rail impedance?

***From similar thread***

What is the expected impedance of +3.3VA_DAC and +5VA rails in below configuration,

1. What is the expected impedance of +3.3VA_DAC power rail in EVK.

2. What is the expected impedance of +5VA rails if we make R46, R56, R58 and R68 are DNI and mount  R114, R120, R127 and R133 resistors

AFE7225EVM-SCH_B.pdf

Please clarify all the points...

Thank you....

  • 0
    TI__Mastermind 18095 points

    FYI...I combined two threads that were asking similar questions.

    There are two considerations to be aware of for the DAC that impact the values and configuration of the load resistors.  One is the Vcm of the DAC should be centered at 3.0V.  The second is that the max bias sink current is 20mA (10mA average).  So, if you want to use a pull-up, pull-down approach then you need to solve this equation.

    Vcm = 3.0V = Vcc - It*R1 = (it - 10mA) * R2

    Indeed, A 71.5 and 165 ohm will do the trick and will keep load voltage at 50 ohms to keep voltage swing within the compliance range.

    It is acceptable to connect the 5V rail to other circuitry.  That will not impact the common mode calculations above.  I do recommend that whatever else is connected to the supply be "clean" so that noise or spurious does not unintentionally affect the DAC output.

    Note, whatever is cascaded after the DAC should be high impedance and can accept a Vcm = 3.0V.  If you are connecting to a modulator as done in the schematics, some consideration is potentially needed to translate the common mode to the required value of the modulator.

    --RJH