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

James Fyne

Part Number: THS4121
Other Parts Discussed in Thread: TINA-TI,

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.

over 5 years ago

0 Herman Theodorus over 5 years ago

TI__Genius 9775 points

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

0 JC Zhu over 5 years ago

TI__Mastermind 29035 points

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

0 Herman Theodorus over 5 years ago in reply to JC Zhu

TI__Genius 9775 points

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