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AD8195 power consumption

Hi,
I have an AD8195 in my design which is powered by a dedicated 3.3V produced by a LDO.  This voltage is only connected to AD8195, decoupling capacitors and some high value pull-ups.  The current measured at the output of the same LDO is 290 mA which is almost twice as much as the maximum possible current I calculated with numbers in your specification.  AD8195 is configured to work with input termination and without preemphasis.  HDMI is working but AD8195 is hot and it happen that my LDO enter current protection mode.  Do you have any idea why the AD8195 would draw more current than what specified?
Thanks
The fallowing table was used to calculate the maximum current.  Current on 5.5 V are set to 0 to calculate the current from 3.3 V supply only.   Maximum current from your specification are used.

            Current     Voltage max

avcc          50          3.3

vtti             54          3.3

vtto            50          3.3

vref_in       0            5

vref_out     0.2         3.3

amuxvcc    0            5

     

Total          154.2

Forwarding to the AD8195 expert

  • Guenter,

         Any progress? We are waiting AD response to release or not.

    Thanks,

    Steph.

  • No answer yet, trying to get in touch with them again right now.

  • Is pre-emphasis enabled, PE_EN = 1? If yes, the total current can approach the values you are observing, especially when the AD8195 TMDS output is floating.  Is the AD8195 output connected to a Rx device?

  • Is pre-emphasis enabled, PE_EN = 1?

    As mentioned in my first post, we don't use preemphasis.  The measured voltage on this pin in 0V (PIN 29).

    Is the AD8195 output connected to a Rx device? 

    AD8195 is connected to a FPGA.  The connection seems OK since we can see video on the output of our project.

    Note:

    Project data flow

    HDMI Connector    ->     AD8195     ->     FPGA     ->     HDMI TX IC     ->     HDMI Connector

  • One possibility is there could be dc current flowing between the AD8195 Outputs and the FPGA SerDes Inputs.  What is the FPGA input termination voltage and common mode voltage range?  Is this connection ac or dc-coupled?  Sorry if this information is in a separate thread. 

    If dc-coupled and FPGA input termination voltage (VDDIO in figure below) is less than AD8195 VTTO (3.3V), there will be dc-current flow between the devices, limited only by the termination resistance.  Series ac-coupling would be recommended in this scenario. 

  • Hi Dave,

    We are in ac coupled configuration.  After your last message we tought we could have a polarization problem since there is no DC path from sink terminaison to AD8195.  We added 200 ohms pull-ups and it worked.  Now the current draw to suplly AD8195 and the new pull-up is 168 mA ( about 18 mA for the pull-ups)  which is a lot better thant 290 mA.  However,  could it be possible to have an explaination about that?  Why does it work now?

    Thanks

    Note: 200 Ohms value was selected so that it doesnt' change the sink termination to much.  (50//200 = 40).   The traces between AD8195 and the FPGA are around 1 cm.

    Charles

  • Hi Charles,

    You result is unexpected, and will require some more digging to find out the root cause.  I'll see if I can reproduce your issue in our lab.  In the meantime, you might consider a larger value resistor than 200ohm, perhaps 1-2kohm to further reduce the change in the impedance discontinuity.

    Dave

  • I have tried unsuccessfully to replicate this issue in our lab.

    I have tried several AD8195 evaluation boards.  Here is my setup.

    Video Source -> Short Cable -> AD8195-EVB -> Short Cable -> Ac-coupled digital crosspoint to mimic ac-coupled FPGA as in your application -> Monitor

    Here is the AD8195 currents I observe:

    TX_EN = 0, PE_EN = 0: 17mA

    TX_EN = 1, PE_EN = 0: 162mA

    TX_EN = 1, PE_EN = 1: 282mA

    The results are consistent board-to-board as well as repeatable after numerous power cycles.

    Could you check the current you observe with TX_EN = 0 as well as PE_EN = TX_EN = 1?