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  • TI Thinks Resolved

TINA/Spice/THS4121: Vocm Impacts Differential Output Peak-Peak Value

Part Number: THS4121

Tool/software: TINA-TI or Spice Models

I'm modeling a THS4121 as a differential-in and differential-out amplifier. I have a gain of 12. Why, when I increase Vocm do I see an increase in the peak-to-peak output swing? The differential output swing (Vout_P - Vout_N) actually increases as Vocm increases, all else being the same. Why would this be?

What brought me to see this is that when I measure (Vout_P - Vout_N) / (Vin_P - Vin_N) the gain, is not 12. The gain value actually varies from around 13 to 15.5. Why the gain differs from 12, and why it appears to vary is puzzling.

  • Hi James,

    I just tried it and I didn't see what you saw. I am using +/-1.65V or (3.3V,0V) supply and I use RF=12k and RG=1k and it works ok as I vary the Vocm pins. It stays constant at gain of 12x.

    Attached is my test bench that I use. THS4121.TSC

    If you need help debugging your test bench or your specific scenarios, can you attach you TINA-TI test bench into the post?

    Herman

  • Hi James,

    Would you please share the model you were using? From your post, it was not clear whether you were using the model TI provided ().


    Thanks,
    JC

  • In reply to JC Zhu - Simulation Models for Product Folders:

    Hi James,

    We have not heard back from you in a while. I'll assume our previous answer has answered your question. I'll close this post, but if you have any further questions, do not hesitate to reply to this post again.

    Herman

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