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

Part Number: ADS8881EVMV2-PDK
Other Parts Discussed in Thread: THS4551

Hi, dear sir / madam:

I have three questions about the User's Guide of ADS8881EVMV2_PDK .

  1. Why the single output voltage of THS4551 is 0.1-4.5V, I think it should be 0.2-4.5V?
  2. How does the 0.1V voltage generate? It is generated by the output voltage rail limitation of THS4551 or other ways?
  3. What do the 4.99 ohm atfer THS4551 use for? I'm really confused about it.

Looking forward to your reply. Thanks so much.

Marc

  • 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

  • Marc,

    Please use the link below for TI Precision Labs: Op amps and scroll down to Section 15.3 addressing FDA stability and Simulating Phase Margin

    Best,

    Luis

    https://training.ti.com/ti-precision-labs-op-amps

  • Hi Luis,

    Thanks for your detailed reply. I will use TINA to simulate the FDA.

    Best regards,

    Marc