Tool/software:
I noticed that TI doesn’t publish input and output resistance values for op amps. Is there a reason? How do I determine the resistances for the LMP7704-SP op amp?
Lucas,
The input resistance is listed for many amplifiers. You can find it in the same section as the input capacitance (see example below for OPA206). If the data sheet does not list the input impedance Marek provides a procedure for finding this value from the simulation model here ( https://e2e.ti.com/support/audio-group/audio/f/audio-forum/1075480/ina134-input-differential-impedance ).
The output "resistance (Ro)" is actually a very important parameter for stability analysis. This parameter is often called "open-loop output impedance (Zo)" in the data sheet. The reason it is called "impedance" is that it may change over frequency. Below is an example of this specification in the OPA206 data sheet. This specification may be shown in the data sheet table, but it is more useful to look at the characteristic curve (see below). Sometimes this is shown as closed loop output impedance (see older data sheets). In this case it is possible to convert to open-loop by dividing by Aol. See Selecting and verifying the driver amplifier for details on how to simulate Zo and how to translate closed loop output impedance to open loop. Note that the vast majority of our amplifier models include an accurate model for input and output resistance.
If there is a specific device you need this spec for let us know and we should be able to help.
Best regards, Art
Lucas,
Fortunately, I do have the Zo (open-loop output impedance) information for the LM770x family. See below. For input impedance the device is CMOS so it is going to be very high (Rcm on the order of 1x10^11). Let me know if this is sufficient.
Best regards, Art
