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.

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