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maximum operating frequency of an op-amp

Sharath krishna kokkula
Prodigy 40 points
Other Parts Discussed in Thread: LM397

Hello,

I am using an op-amp comparator circuit in my design.

How do I know the maximum operating frequency of an op-amp.

thank you.

  • 0
    TI__Genius 14317 points

    Hello Sharath,

    It's not good practice to use an op amp as a comparator because the output stage could saturate when driven to either supply rail when switching. A rail to rail output OA helps with that problem, but an external hysteresis resistor is still required. A much better design practice is to use a device like the TLV101/2/4 which has built in hysteresis and a host of other desirable comparator features like OV/OC detection, zero crossing detection, high input common mode voltage range, wide input range, open collector output etc. The comparator "speed" is determined by the propagation delay spec, output slew rate, load and other parameters. One rule of thumb that I've used is that maximum f should be less than 1/(6 x tpd).

  • 0 in reply to Jeffery Pfarr
    Prodigy 40 points

    thank you for your reply..

    I selected LM397 as comparator but in that datasheet there is no slew rate,rise time,gain band width product..

    in the datasheet mentioned TPHL =  900ns(Max)  TPLH= 940 ns  I calculated max frequency asbelow

    total propagation delay=900+940=1840ns

    as per your guidance delay=6*1840 ns=11.04usec

    frequency=1/(11.04u sec)=90.58KHz.

    is it right pls confirm..

    thank you once again...

  • 0 in reply to Sharath krishna kokkula
    TI__Genius 14317 points
    Hello Sharath,

    Now that I know more about your application and that you are using the LM397, I will forward this thread to the team that supports this device for their expert commentary. Thanks for using the TI E2E Forums.
  • 0 in reply to Sharath krishna kokkula
    TI__Guru 50236 points

    Hello Sharath,

    The LM397 is a comparator, so it will not have slew rate and gain bandwidth product specifications.

    What you are looking for is called the "Maximum Toggle Frequency".

    One rule of thumb is 1/(6*Tresp), which tends to be very pessimistic.

    The more specific way uses the sum of the risetime, fall time, and the tplh and tphl specifications.

    Assuming the rise and fall times are 150ns each:

     Ftoggle = 1/(150ns+150ns+900ns+940ns) = 1/(2.14us) = 467kHz

    I would say about 400kHz to give it some margin. Be aware that the input level will also have an effect, due to the overdrive vs. response time effects (Figs 4 and 5).

    Output capacitance will severely effect the rinse and fall times due to the RC time constant, particularly during the rise-time - so use a lower value pull-up resistor (say 1K) and minimize the output capacitance.