ADS8881EVMV2-PDK: Questions about User's Guide of ADS8881EVMV2_PDK

Hello Marc,

The THS4551 output swing is dependent on the load condition and temperature; and as you have mentioned, this is dependent on the output stage of the THS4551.  The datasheet provides a typical 0.2V swing from the negative rail at 25C when using a 1kOhm load resistor.  In the circuit above, the load is an RC filter and not a constant 1kOhm load, so at low frequencies or DC signals, the output stage of the THS4551 sees high impedance and will be typically able to swing closer to the rail.  If you want to use the most conservative approach across temperature and input frequency variations, the most conservative approach may be use 0.23V-4.5V.

The 4.99 Ohm resistor inside the feedback loop is used to increase phase margin and therefore improve stability and settling of the circuit while driving the R-C-R filter (4.99ohm-10nF-4.99ohm). Since the 4.99 ohm resistors are inside the feedback loop, they do not contribute to the Gain error, and the gain of the FDA is only function of the ratio of the 1kOhm feedback resistors and 1kOhm input resistor.

A detailed session on FDA stability analysis using TINA simulation can be found on the TI Precision Labs: Fully Differential Amplifiers: FDA Stability and Simulating Phase Margin. 

Thank you and Best Regards,

Luis