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THS4140: Distorted output above +/- 3V

Russell Wetherley1
Expert 4255 points
Part Number: THS4140

Hello E2E,

I need to be able to test a products video input to ensure it will tolerate a +-5V common mode voltage. This is analog video so think of an NTSC  video type of signal (1.2V p-p), but it is input to the product as differential.

I selected the THS4140 fully differential amplifier since it has the OCM pin. I run the THS4140 from +-12V supplies. I connect a separate supply to the OCM pin for +-5V variation capability. I have the amplifier set for gain of 1 and have tried both 390 OHM and 1K OHM resistors for Rf and Rg (your data sheet ref des).

I use a NTSC signal generator and run the circuit as single end to differential conversion.

When I look at the output I get a nice differential version of the input over a common mode range of ~0+-3V. The output varies up and down nicely while I vary the VOCM source as expected. But as I go above +-3 V, the output starts to distort and by about 4.2V the output is completely smashed. The outputs continue to follow the OCM voltage up to 5V(where I stop), but the signal is killed off.

What am I missing? I figured as a crude estimate that 5V+1.2V is <7V and so +-12V supplies should have way more than enough "head room". I even tried running at +-14V and that didn't make any difference.

I am basically running the circuit from figure 30 of your data sheet, with an additional 75Ohm termination resistor to ground across the input signal source. I dont see the input source begin affected other than the smaller Rf and Rg values tended to "pull it up or down" slightly.

So I need some help getting this running to the full +-5V without distorting the output. Also out of time for my test phase.

Thank you for the help!

Regards,

Russell

  • 0
    TI__Genius 15975 points

    Hi Russell,

    Do you know what is the product's input impedance? Is it 150-ohms differential? What is the maximum signal frequency of the NTSC carrier you are testing with? Is it 4.5MHz? Also, are you measuring the distorted waveform using a scope probe? Have you made sure that the scope channel is set to 1Mohm input? I think based on what you explain as the THS4140 circuit configuration and running on +/-12V supply, the part should be able to support +/-5V output common mode range.

    I would first try disconnecting the THS4140 output from the product's video input, and make sure that there is no undistorted signal waveform by varying the VOCM to +/-5V.  I would also keep the Rf and Rg at 1kohm just so that the amplifier does not load the signal generator. I would also recommend sweeping the THS4140 inputs with single-tone frequency keeping the amplitude constant and see if there is any noticeable change in the output signal amplitude along the +/-5V VOCM. The frequency sweep has to be in the same neighborhood as the NTSC signal frequency.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    TI__Expert 4255 points
    Hello Rohit,
    This is for a customer of mine and here is what we know thus far:

    Do you know what your input impedance is? Is it 150-ohms differential? Answer: 75 Ohm single ended. I have NOT tried running with a thevenin equivalent on the grounded input leg yet as mentioned in one of your app notes on single ended to differential video conversion (by Jim Karki). But it was trying to get rid of a common mode offset, I am trying to inject offset.
    What is the maximum signal frequency of the NTSC carrier you are testing with? Is it 4.5MHz? Answer 4.7MHz
    Also, are you measuring the distorted waveform using a scope probe? Answer : Yes
    Have you made sure that the scope channel is set to 1Mohm input? Answer: 10Mohm.

    I did try running the THS4140 with no load, but it oscillates a lot without a load.
    I also did a set of tests watching how the differential video signal was affected as I powered up/connected/ disconnected the signal at various points along the path. I discovered there is an analog switch in between my card and the diff amp (on another piece of test gear). It may be the root of the problem since it is only powered form +3.3V and in looking at the data sheet I dont think it has the "headroom" to work over the common mode range I need.


    Russell