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LMV225: Large variation in measured power (8dB)

Justin Miller
Prodigy 10 points
Part Number: LMV225
Other Parts Discussed in Thread: LMV226, LMV228

We're using the LMV225 (WSON) to measure L-Band RF power.  We're seeing a lot of variation, chip to chip in the output voltage. 

We use a WB-SG1 signal generator to create a carrier at 1.4 GHz at a calibrated power level.  The measured signal (Using an HP 427B power meter) is -23.5dBm.  We're using a PCB trace 20dB directional coupler to sample the signal, and a 16dB (at 1.4GHz) gain amplifier (MAAL-010704) to bring the measured signal within a good range for the LMV225.

We're finding that the measured output voltage varies from 0.65V to 0.93V. (-32dBm to -24dBm)  We've swapped MANY LMV225 chips between boards, and the measured voltage follows the LMV225 chips, not the boards.  Swapping the amplifier has no real effect, so it seems stable.  We've been very careful not to exceed the solder temperature of 260°C.

The 8dB variation in measured power is proving to be a big problem. We've had to replace a lot of the outlier LMV225 chips to bring their measured power close to the average.  Not a workable solution going forward.

We're using the recommended circuit from the data sheet.  Attached is our schematic.

Any help would be appreciated.

  • 0
    Guru 140200 points

    Hi Justin,

    have you read this thread?

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1066931/lmv225-lmv225-anormal-output-voltage-spread

    Have you checked the circuit with C65 = 100...220pF?

    Can you post your layout?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 10 points

    I did read that thread.  The only possible thing I saw in it was that they weren't grounding the center pad, but we are. 
    We are using a 100n cap for C65.  I just ran a couple tests with the calibrated input signal described above. The LMV225 in Board 1 has a much lower reading than average.  I've seen a few of these since I did the first batch of tests.

    Board 1, C65=100nF, output: 0.589V
    Board 1, C65=100pF, output: 0.583V

    Board 2, C65=100nF, output: 0.827V
    Board 2, C65=100pF, output: 0.820V

    Changing to 100pF caps didn't help.

    The signal comes in through C61.  It's amplified by U7.  Passes through C65, and into U8 (LMV225).  There's a full ground plane underneath.

  • 0
    TI__Expert 3390 points

    Hi Miller,

      Datasheet specifies LMV225 for resistor tap based application and LMV226/LMV228 for applications with directional couplers. I see you are using LMV225 with directional coupler. 

           I will check internally and get back to you on this issue.

     Please capture output of LMV225 using oscilloscope and see if there are any oscillations. 2-3 devices data would be helpful.

    Also can you share the signal chain schematic from directional coupler to LMV225.

    Regards,

    Hruday

  • 0 in reply to Hruday Kumar Mudireddy
    Prodigy 10 points

    Hi Hruday.
    The input range wouldn't work for the LMV226/228 in our application.  We need the lower range of the LMV225.

    Here are the scope views of the 2 boards tested above. I didn't see any ripple in any time scale.

    Board 1

    Board 2

    Here's the whole layout of the RF section.  Signal comes in over the LNB connector.  The Modem connector is terminated with a 75Ω terminator.  The larger coupled trace is 50Ω.  We're not seeing variation in any parts of the circuit other than the output of the LMV225.

  • 0 in reply to Justin Miller
    TI__Expert 3390 points

    Hi Miller,

       Thanks for the quick answers.

     Currently I couldn't access the production data.

    To check this can you help me with below:

    1. Take 3 boards, one with least out voltage, one with average and one with maximum out voltage.

    2. Read the out voltages for two power levels one at -23.5dBm and at -13.5dBm and let me know the reading for these three boards.

    Thanks & Regards,

    Hruday

  • 0 in reply to Hruday Kumar Mudireddy
    Prodigy 10 points

    Thank you for the quick replies also Hruday.

    I tested three boards.  I don't have one of the max output ones handy, but the third is a bit higher than average.  I also measured the voltage with a terminated input.

    Board 1
    terminated input: 0.174 V
    -23.5dBm input: 0.585 V
    -13.3dBm input: 1.038 V

    Board 2
    terminated input: 0.189 V
    -23.5dBm input: 0.833 V
    -13.3dBm input: 1.262 V

    Board 3
    terminated input: 0.196 V
    -23.5dBm input: 0.891 V
    -13.3dBm input: 1.312 V

  • +1 in reply to Justin Miller
    TI__Expert 3390 points

    Hi Miller,

        Thanks for the data. 

    All 3 devices have Kslope almost equal and in alignment with datasheet.  For terminated input, Output Voltage can vary upto 300mV from below table in datasheet

    So I think is 8dB of variation is expected from this device. You can address this by single point calibration on your system. As the slopes are not varying much one point calibration at any known input power should solve the problem.

    Thanks and Regards,

    Hruday

  • +1 in reply to Justin Miller
    Guru 140200 points

    Hi Justin,

    unfortunately, logaithmically working amplifiers usually suffer from considerable manufacturing tolerances. If highest precision is wished, a calibration is strongly recommended. In any case, I would carry out a calibration of offset voltage (with no RF input). This could even be accomplished purely analogue.

    Keep in mind, though, that for many of the specs (logarithmic slope, etc.) only typicals are given in the datasheet. So, even a more sophisticated two point calibration (e.g. by a µC) may be desirable.

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 3390 points

    Thanks Kai for your response!

    Miller,

        I am going ahead and mark this response as TI Thinks resolved. Fell free to reopen the thread if you need further support.

    Regards,

    Hruday

  • 0 in reply to Hruday Kumar Mudireddy
    Prodigy 10 points

    Thank you for your help Hruday.  I had thought this would work without calibration, but it looks like calibration is necessary.