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TLC2262 Output impedance seems high

TIguru
Expert 1430 points
Other Parts Discussed in Thread: TLC2262, OPA2378

TI team,

  I have a question question concerning TI's TLC2262 op amp. In the datasheet, the output impedance is indicated to be 250 ohms and I would like to know whether this is a typo or is it accurate. Typically Op Amps have much lower output impedances, hence the question. If it is indeed 250 ohms, can you recommend a similar device with lower output impedance? In advance, thank you for helping.

 

Cheers,

TG

  • 0
    TI__Genius 11680 points

    TG,

    First, I don't think it's a typo. Figure 17 pretty clearly shows about 230ohms of output impedance.

    How much output impedance and what frequency are you looking for? It looks like OPA2378 would be better, but I don't know how to compare closed-loop output impedance with open-loop output impedance.

    Can anyone else comment on that?

  • 0
    TI__Guru 62925 points

    Hello TG,

    The 250 ohm value is most likely correct for this type of op-amp.  If you look at other TLC22xx parts the zo values are close to the same. 

    In general for op-amps with CMOS output stages, the open-loop output impedance can be approximated by Zo ~ 1/(2*sqrt(k*Id)).  In the unloaded case, the drain current, Id is only the class AB bias current that flows through the output transistors and can be approximated by Iq/2, where Iq is the quiescent current of the amplifier.  Therefore as the quiescent current of the amplifier decreases, the output impedance increases.  The TLC2262 has a quiescent current of 500uA so I would expect the output impedance to be in the few hundreds of ohms based on the Iq current. 

    It should be noted that the older TLC datasheets plot the closed-loop output impedance and not the open-loop output impedance.  The open-loop output impedance is the curve you will need to determine what effects different load impedances will have on the stability of the circuit.  The closed-loop output impedance is equal to the open-loop output impedance divided by the loop gain.

    Zcl = Zol/(1+AOLB).  Where AOL*B is the loop-gain.

    Please let me know if there are additional questions.

    Best Regards,
    Collin Wells
    Precision Analog

     

     

     

  • 0 in reply to Collin Wells
    Expert 1430 points

    Collin

       Thank you for the thorough explanation.  You've answered my questions.

     

    Best regards,

    TG