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LMH6502 - Output gain does not match to the programmed gain setting

Kawai
Guru 19775 points
Other Parts Discussed in Thread: LMH6502

Hi Team,

I had built the LMH73033EVM using LMH6502. The attached is the schematic and the measured waveform using the EVM. I had programmed the gain to Av=4,37 however, the gain was only Av=2.2.

Could you please advise us what is wrong with my schematic and how we could get the expected output ?

LMH6502 setup on LMH730033EVM 20150819.pdf

Best Regards,
Kawai

  • 0
    TI__Mastermind 22825 points

    Hi Kawai-san,

    The LMH6502 voltage gain is defined to be from the differential input to the output (pin 10). From what I can gather, your "Vout" is at the 50ohm shunt load and there is a 50ohm Rout in series with the output (which is a 2:1 voltage divider).

    I'm thinking that this voltage divider could account for you measuring 1/2 the gain you had expected?

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Guru 19775 points

    Hello Hooman-san,

    Thank you for your prompt reply and sorry that this seemed to be a simple problem. I will try monitoring the device output before 50 ohm series resistor.

    This series resistor is shown in the datasheet for single supply typical application schematic (page 19, Figure 61). Is this application example for 1/2 of the programmed gain setting monitoring directly to the oscilloscope with 50 ohm SMA cable ?

    Best Regards,
    Kawai

  • 0 in reply to Kawai
    TI__Mastermind 22825 points

    Hi Kawai-san,
    The 50ohm series output resistor is a desirable thing to have when probing the output as it isolates any cabling / connector parasitics and provides a 100ohm (=50+50) load (which is how the LMH6502 was characterized, mostly).
    Just factor-in the fact that there is 6dB loss of signal from LMH6502 output pin to the 50ohm scope input.

    Regards,
    Hooman

  • 0 in reply to Hooman Hashemi
    Guru 19775 points
    Hi Hooman-san,

    Thank you for your excellent support.

    Best Regards,
    Kawai