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Open loop output impedance of INA129 and INA121

Other Parts Discussed in Thread: INA121, INA129, TIPD128


1. What will be the open loop output impedance of INA129 and INA121?

2. Is there a way to use the INA spice model and extract the model of 3 opamps inside these INAs seperately to check the stability?.

3. How much the gain bandwidth (1.3MHz and 600KHz respectively for INA129 and INA121 at Gain=1) will change over the operating temperature?




  • Hello Shihab,
    We are looking into this and will get back with you soon.
  • Hello Shihab,

    For the INA129, even though the simulation model does not explicitly state that open loop output impedance is modeled, the simulated value (~300 ohms) is relatively close to our estimated calculation (110-200ohms).

    For the INA121, however, the simulated closed loop output impedance (1ohm) is nowhere near our estimated calculation (150-300ohms).

    After looking at the design of the INA129 and INA121, a colleague of mine estimates that the bandwidth of the aforementioned devices may vary by approximately +/-10% over the operating temperature range. Note that the bandwidth can also vary over process. We estimate that variation to be approximately +/-30% ( Please understand that these are approximations. You will have to fully characterize your design with many samples over temperature to ensure robustness.

    You may want to consider buffering the output of your INA with an op amp. TIPD128 is a reference design that provides isolation resistor values for various capacitive loads for a number of different op amps.