I'm trying to calculate slew rate of OPAMP.

  1. Applied square wave with 3V peak value to VIN+ of OPAMP.
  2. Connected VIN- to VOUT as a feedback.
  3. Graphic of VOUT voltage can be seen below.

enter image description here

  1. Calculate Peak value of VOUT.
  2. Find the time value of %90 (Time%90) and %10 (Time%10) of VOUT.
  3. SR = (Vpeak*9/10-Vpeak*1/10)/(Time%90-Time%10)

I want to ask two basic question:

  1. Is my slew rate calculation TRUE?
  2. My OPAMP has the values of VDD +5V and VSS -5V. So should I apply square wave to OPAMP with 5V peak value? Or is it enough to apply square wave with 3V peak value.
  • \$\begingroup\$ I don't think that's right... you'll be conflating GBW and SR. Just hit your opamp with a big (fast) step and measure how fast the output voltage rises... Volts/ uS. (They also list the value in the spec sheets... typically.) \$\endgroup\$ Jan 10, 2017 at 16:16
  • \$\begingroup\$ So how can I calculate Slew Rate? \$\endgroup\$
    – netizen
    Jan 10, 2017 at 16:18
  • \$\begingroup\$ Do you have a 'scope? You can read the slew rate directly, as the rate of rise when you hit your opamp buffer with a large amplitude square wave. \$\endgroup\$ Jan 10, 2017 at 16:21
  • \$\begingroup\$ I'm using Pspice to simulate \$\endgroup\$
    – netizen
    Jan 10, 2017 at 16:22
  • \$\begingroup\$ Well then plot the voltage vs. time... (just like a 'scope). I don't know much about opamp modeling in spice, but isn't the slew rate one of the input parameters \$\endgroup\$ Jan 10, 2017 at 16:27

2 Answers 2

  1. What you did is OK.
  2. You don't need to drive the Opamp into saturation in order to measure the slew rate, I would even say that it is not as ok as what you did.
  • \$\begingroup\$ "I would even say that it is not as ok as what you did" I dont understand this? \$\endgroup\$
    – netizen
    Jan 10, 2017 at 15:43
  • \$\begingroup\$ I mean that applying a square wave of 5 peak value may give you a less precise measure, because of the saturation effects. \$\endgroup\$
    – diegobatt
    Jan 10, 2017 at 15:47
  • \$\begingroup\$ Here are the basic requirements for slew rate simulationn: (1) unity gain feeedback and (2) a step input (as steep as possible) with an amplitude that drives the first stage into saturation (but NOT the ioutput) until the feedback signal brings this stage back to linear operation. Hence 1 volt would be OK, but 3V are the upper limit. \$\endgroup\$
    – LvW
    Jan 10, 2017 at 17:12

Slew rate limiting is internal to the op-amp of course and may be different for bipolar, small, large signal or pulse responses and for all or a portion of the output signal. Depending on your +/-V supplies your amplifier may be slew rate limited for only a portion (not necessary 10%-->90% as you use for rise time measurements). In any given practical amplifier you can measure the slew as you propose with an oscilloscope, but to do amplifier selection based on a slew rate capability can be considerably more complex. This article may help, but many of the high bandwidth high slew rate chip providers have application notes covering their view of slew rate measurements.

  • \$\begingroup\$ Ok. Understand that article. When I apply different square waves, obtain differrent slew rate. I am confused about which one is the optimum slew rate. \$\endgroup\$
    – netizen
    Jan 10, 2017 at 16:53
  • \$\begingroup\$ Whether the slew rate limiting you measure is applicable to your particular application only you can determine. It's not a case of being told which is likely optimal. \$\endgroup\$ Jan 11, 2017 at 6:38

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