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TLK1501: TLK1501-Correct value of RREF value

Mehmet Nazim Batur
Prodigy 30 points
Part Number: TLK1501


Hi all,

We would like to use TLK1501 in 50 ohm environment. Could you please answer the questions given below?

  1. RREF is given as 200 ohm in the datasheet. However, 100 ohm resistor is used in the EVM circuitry.  (Please see attached.) Which one is correct?
  2. We will use two TLK1501 ICs on seperate PCBS, one of them will be transmitter and the other one will be receiver. We'll use 50 ohm cables for connection. Which one is the best choice for our application, AC coupled or DC coupled circuitry? What is the difference between them?
  3.   Could you please share min/max values for VOD(p) for AC coupled circuitry? (It is given as DC coupled version in the datasheet page 18.)

Regards,

 

 

TLK1501_EVB_screenshot.jpg

 

  • +1
    TI__Mastermind 39543 points

    Hi Mehmet,

    1. I would recommend following the data sheet recommended value of 200 Ohms.

    2. AC coupling is common with high speed interfaces.  This allows for devices with different termination bias voltages to be connected together.  Also, this helps eliminate the data lines as a path for DC current that might occur in a ground imbalance.  I would recommend AC coupling.

    3. Unfortunately I don't have this data available.

    Thanks,

    Drew

  • 0 in reply to Mehmet Nazim Batur
    Prodigy 30 points

    Hi again Drew,

    As you recommended we use AC coupling option with 200 ohm RRFEF. However, there is a communication error. If we use 100 ohm instead, communication is OK.

    We're measuring output voltage waveform wih oscilloscope and VOD(pp_p) is around 300mV for 200 ohm RREF. It is around 800 mV for 100 ohm RREF. I think this error comes from the inappropriate value of VOD(pp_p). 


    What should be the VOD(pp_p) value? (min or max)

    Do we need to see similiar waveform on oscilloscope for reliable communication?(for AC coupling) 

    P.s: Internal loop-back test is OK for both version, 200 ohm and 100 ohm. 

  • 0 in reply to Mehmet Nazim Batur
    TI__Mastermind 39543 points

    Hi Mehmet,

    Apologies for the delay on this.  After some discussion with some colleagues, I think I can help clarify your observations.

    You may have noticed that there is a difference in the data sheet between DC coupled and AC coupled mode (figures 15 and 16).  Specifically, DC coupled lack Rt on the TX side of the device.  This comes from a trade-off between power and return loss.  The AC coupling topology has improved return loss, but requires increased bias current in order to achieve the same amplitude.  Specifically, we expect the DC coupled case to have 2x the amplitude relative to the AC coupled case for the same bias current since the DC coupled case lacks Rt on TX.

    By reducing the RREF, you are increasing the bias current.  This increases VOD to be more comparable to DC coupled case.

    Looking back at our initial recommendation for 200 Ohm, I think the data sheet focuses more on the DC coupled case, which is where the recommendation came from.  I think there are a few data points that support using the 100 Ohm for AC coupled use case:

    • EVM uses AC coupled topology and uses 100 Ohm
    • We understand that AC coupled topology will reduce amplitude, so increasing bias current to compensate makes sense.
    • You are observing good communication with 100 Ohm

    Thanks,

    Drew