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CDCV304: output impedance

Joyce Li
Intellectual 2170 points
Part Number: CDCV304

Hi Dear expert,

My customer is asking about CDCV304 output impedance when VDDIO=3.3V.

Would you please help support this?

Thanks a lot!

BR,

Joyce

  • 0
    TI__Mastermind 28540 points

    Hello Joyce,

    Figure 2 and Figure 3 of the datasheet show V vs I plot for high and low level output voltage.  From these plots we can calculate the output impedance as the slope of the line. 
    Figure 2 & 3 are for a Vdd (Vddio) case of 3.3 V.

    Looking closer to the lower current in/out, we can see the line is very linear and is approximately 25 ohms by my rough calculation, as current increases into a higher load we can see the output impedance does increase a bit.

    73,
    Timothy

  • 0
    TI__Intellectual 2170 points

    Hi Timothy,

    Thanks for reply. I still have questions and double confirm with you:

    1. For Figure 2, it is high level output voltage VS high level output current, the current is negative value, do it mean when output high level, i.e. VOH>=2.0V, the current is sinking?

     2. For Figure3, the output current is lower, does it mean the load is higher impedance and load is lower impedance, the output current is higher? So you estimate the output impedance with higher current, say 2.5V, 100mA? 

    I am not sure my understanding is correct, so confirm with you.

    thanks a lot for your reply in advance.

    Joyce. 

  • 0 in reply to Joyce Li
    TI__Mastermind 28540 points

    Hi Joyce,

    Joyce Li said:
    1. For Figure 2, it is high level output voltage VS high level output current, the current is negative value, do it mean when output high level, i.e. VOH>=2.0V, the current is sinking?

    By convention, current into the device is positive, for sample current consumption of the device.  So the negative current means the device is sourcing current.

    Refer to figure 4 for the test figure.  Note it is driving a load that is a 140 ohm - 140 ohm voltage divider with a 10 pF capacitor.

    Joyce Li said:
     2. For Figure3, the output current is lower, does it mean the load is higher impedance and load is lower impedance, the output current is higher? So you estimate the output impedance with higher current, say 2.5V, 100mA? 

    It is the same current - but it is mirrored to the Voh spec.  So when the voltage is high on the pin but a low is commanded, then it sinks a lot of current trying to get the output to go low.

    It's simplest to pick two points on the curve to estimate the impedance in the general range of operation.

    73,
    Timothy