• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 49 subscribers
  • Views 637 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

LMH6518 output on oscilloscope

LMH6518: LMH6518 overdrive in DSO application

Jeff Xie
Intellectual 1780 points
Part Number: LMH6518

Hello,

My customer is using LMH6518 in the 500MHz bandwiidth DSO application. A large offset (hundreds of mV) is observed when the output of LMH6518 is over drived under 500MHz. Is this normal? What can we do to solve this problem?

Thanks

  • 0
    Guru 140200 points
    Hi Jeff,

    what do you mean by "over drived"? That the built-in overvoltage clamping sets in?

    Kai
  • 0 in reply to kai klaas69
    TI__Intellectual 1780 points

    Hello Kai,

    The input and output are DC coupled, single ended input differential output. VIN+ is used as a signal input, and VIN- is a fixed 2.5VDC. VCM=1.25V. Put a 2.5V Offset, 560mVpp, 500MHz signal into VIN+ and measure the output of LMH6518.When 6518 is configured as 0x40a (LG -1.14dB), 6518 output is normal.

    But when I  configure HG  =34.86dB, there is a large offet, and the ADC can not sample any AC signal. 

    Figure 1 0x417（HG  24.86dB）

    Figure 2  0x413（HG 34.86dB）

  • 0 in reply to Jeff Xie
    Guru 140200 points
    Hi Jeff,

    have a look at figure 59 of datasheet: In "HG" mode the pre-amp stage has a gain of 30dB. With an input signal of 560mVpp this would result in a 17.7Vpp signal at the output of pre-amp which is just impossible. So, the pre-amp is totally overdriven! The cure is to not set the pre-amp into "HG" mode but only "LG" mode, if your input signal is such high.

    Kai
  • 0 in reply to Jeff Xie
    TI__Genius 15975 points
    Hi Jeff,

    As Kai mentioned, the front-end pre-amp gain stage is over-driven when applying 560mVpp at HG setting and it could potentially be the reason for the large output offset voltage. Is there any reason for using the HG setting instead of LG setting with 560mVpp signal? You may want to refer to page 31 in the datasheet for better understanding of the clamping feature.

    Best Regards,
    Rohit