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LMH6554LE-EVAL higher speed differential amplifier

Feihu Xu
Prodigy 20 points
Other Parts Discussed in Thread: LMH6554, LMH6554LE-EVAL

We bought a Differential Amplifier evaluation board as "LMH6554LE-EVAL higher speed differential amplifier evaluation board". The number on board is "551600216 - 002 LMH6554 Diff Amp LLP 14 EVAL board". After we tested the board, it has two problems:


1. The Gain. On the data sheet, it says the board has a factor of 6dB amplification, but when we test it, the gain turns out to be -3dB, which means Vout/Vin=1/2. 

From the analysis of the amplifier circuit on the board, the gain should be R_{F_M}/R_{G_M},. We also tested R_{F_M} and R_{G_M} as R_{F_M}=171 and R_{G_M}=85, which should be 6dB amplification. But, the signal we got is only -3dB amplification. The gain is not 6dB.

So, any ideas on the gain problem?

2. Single-ended output has no signal coming out. We test the board with Vcc=+2.5 and Vee=-2.5, and use single input (a typical SIN function) to Vin+ and ground to Vin-. The outputs, Out+ and Out-, do have signal coming out, which is -3dB factor of the input SIN function as discussed above. But, the single-ended output "Out" always has no signal 

So, what is wrong with the single-ended output? Does the transformer have some problems?

I hope some experts can help us regarding this board. Thank you very much!

  • 0
    TI__Mastermind 22825 points

    Hi,

    The LMH6554 EVAL board BOM (link below) shows that RF=200ohm, and RG=91ohm when you get it. With this default setup, the gain (from Vin+ to differential output) should be 6dB.

    http://www.national.com/an/AN/AN-1945.pdf

    The Single ended output components (R12, R13, C14, and C15) are normally not stuffed on the board. That's why you have no output at "OUT" on the transformer. Stuff these components to get a single ended output.

    Note that differential output (at OUT+ and OUT-) has 50ohm (R6, R7) series output and will therefore be attenuated if you use a 50ohm instrument to read them.

    Also:

    1. VIN+ should be an AC coupled, 50ohm input source and Vin- should be left floating (not shorted to ground). If your source is DC coupled (50ohm), short across C2 for balance.

    2. Gain definition is from input (Vin+) to differential output (pins 12 and 13).

    3. With the +/-2.5V supplies you have noted, please ground VOCM for testing (to center the CM output voltage)

    Let me know if you still need some more help.

    Thanks,

    Hooman Hashemi

  • 0 in reply to Hooman Hashemi
    Prodigy 10 points

    application circuit which is provided in the datasheet will be able to amplify the input signal or i have to do put some extra component to make the thing work properly .

  • 0 in reply to sam naik
    TI__Mastermind 22825 points

    The EVAL board that you have received (as is) can provide an amplified output provided you ensure the following:

    1. VIN+ should be an AC coupled, 50ohm input source and VIN- should be left floating (not shorted to ground). If your VIN+ source is DC coupled (50ohm), short across C2 for balance

    2. With the +/-2.5V supplies you have noted, please ground VOCM SMA for testing (to center the CM output voltage)

    3. Measure individual outputs at OUT+ and OUT-. If you like to get a single ended output at the OUT SMA instead, then you need to stuff R12, R13, C14, and C15

    Thanks,

    Hooman Hashemi

  • 0 in reply to Hooman Hashemi
    Prodigy 20 points

    Thanks a lot for the reply. If I want to use the single-ended output, what are the values of R12, R13, C14, and C15 that I have to stuff? I didn't find the suitable values regarding these four components on the specification. Can I simply shorten R12, R13, C14, and C15?

  • 0 in reply to Feihu Xu
    TI__Mastermind 22825 points

    Hi,

    The values are not very critical. The total load on LMH6554 output(s) will be: R6 + R7 + 4*RL or about 300ohm which is fine (as stated in AN-1945), assuming R12= R13= C14= C15= 0ohm, R6= R7= 50ohm, and your RL is a 50ohm test equipment load. This is assuming that your are using a 4:1 transformer for T1 (e.g. TC4-19).

    Some of this is discussed on page 1 of AN-1945:

    "If single-ended output is desired an output transformer such
    as the TC4-19 from Mini Circuits can be utilized. The TC4-19
    has a 4:1 impedance ratio (2:1 turns/voltage ratio). This is
    particularly useful for interfacing to a 50Ω test equipment.
    When referencing the transformer datasheet, the
    LMH6554LE−EVAL evaluation board has the primary windings
    on the output side of the evaluation board and the amplifier
    is driving the secondary windings. This provides a step
    down transformation from the differential amplifier output to
    the test equipment. The center-tapped secondary winding also
    allows a differential to single ended conversion (Balun).
    The impedance seen by the differential amplifier = (R6 + R7 +
    RL*4), where RL is the impedance from pin 4 of the transformer
    to the load. For example, if RL = 50Ω for the test
    equipment, to achieve an impedance of 500Ω seen by the
    LMH6554 differential output R6 = R7 = 150Ω with C14 = C15 =
    R12 = R13 = 0Ω. The LMH6554LE−EVAL board is equipped
    with pads to add additional filtering schemes using C14 - C18
    and R8 - R13."

    Hope this helped.

    Hooman