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ADC-WB-BB: application

Ali Sorouri
Prodigy 90 points
Part Number: ADC-WB-BB

Hi,

I have purchased two of these converters to measure the insertion loss of twisted pair telephone wire at different length. The result I am getting does not look anything I expect. For my testing I use a network analyzer (a spectrum analyzer with signal source that can be used as network analyzer). I use the normalization feature of the analyzer to zero out the effect of these BALUNs and expect to see no or very little attenuation and almost a flat response up to 200MHz for short length and gradually get attenuated with longer length of wire pairs.

Should I remove the two 49.9Ohm resistors on both of these BALUNs?

  • 0
    TI__Mastermind 42220 points
    Hi Ali
    The balun used on this board is a Mini-Circuits TC1-1-13MA+, which has 1:1 impedance ratio.
    In this application we intend to have a 50 ohm single ended input at J1 and two 50 ohm single ended outputs at J2 and J3 which are opposite in phase. J2 and J3 are intended to be connected to the VIN+/VIN- 50 ohm single ended/100 ohm differential inputs of a number of different ADC evaluation boards. The ADC12D1800RFRB is one example.
    If your effective load impedance at J2/J3 is not 100 ohm differential then you may get different results than expected due to the impedance mismatch between the board and your transmission line.
    Best regards,
    Jim B
  • 0 in reply to Jim Brinkhurst1
    Prodigy 90 points

    LabWireLineAttenuation.pdfThanks for the reply Jim.

    My application is very simple. I have two of these units connected back to back J2 to J2 & J3 to J3 using twisted pair wires to maintain the coupling throughout the length of the wire. The J1 connectors of these devices are connected to the test equipment set for 50 Ohm input and output.  

    I was expecting to see almost 0-dB loss for short wire like 20ft but the loss I am seeing is way too much and the response is not flat or uniform drop off with increasing frequency. See attached result. I am wondering if ADC-WB-BB is the right converter for this testing.

    thanks again

    Ali

  • 0 in reply to Ali Sorouri
    Prodigy 90 points
    I mistakenly clicked the "This resolved my issue" and could not undo it. So I clicked reply to continue.
  • 0 in reply to Ali Sorouri
    TI__Mastermind 42220 points

    Hi Ali

    I did some quick measurements of known good boards connected back to back as you have. I used short 6" SMA-SMA RF cables in between the balun boards.

    With 0dBm applied to the input, these are the measured output powers versus frequency for 1 to 200 MHz:

    Frequency (MHz)

    Measured Power (dBm)

    1

    -3.85

    5

    -3.68

    10

    -3.87

    20

    -3.96

    30

    -4.14

    40

    -4.39

    50

    -4.72

    60

    -5.03

    70

    -5.43

    80

    -5.75

    90

    -6.06

    100

    -6.42

    110

    -6.72

    120

    -6.98

    130

    -7.22

    140

    -7.39

    150

    -7.62

    160

    -7.79

    170

    -8.02

    180

    -8.2

    190

    -8.39

    200

    -8.59

    As you can see the insertion loss is around 3.9 dB at low frequency and increases to around 8.6 dB at 200 MHz.

    These balun boards were developed to provide a cost-effective solution for customers needing a way to drive our differential input ADC evaluation modules. They weren't intended to be the highest performing solution available. If you need something that is very flat across frequency then a lab quality product similar to this would be more appropriate:

    http://www.hyperlabsinc.com/HL9402.aspx

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 90 points

    Hi Jim,

    Thanks for the reply and the measurement. I figured that pairs of coax cable that I was using to connect the DUT (which is a twisted copper wire plant) needed their GND breaded connected to frame ground of DUT. Now things are looking better. 

    Ali