1
\$\begingroup\$

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.
\$\endgroup\$
  • \$\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\$ – George Herold Jan 10 '17 at 16:16
  • \$\begingroup\$ So how can I calculate Slew Rate? \$\endgroup\$ – netizen Jan 10 '17 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\$ – George Herold Jan 10 '17 at 16:21
  • \$\begingroup\$ I'm using Pspice to simulate \$\endgroup\$ – netizen Jan 10 '17 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\$ – George Herold Jan 10 '17 at 16:27
2
\$\begingroup\$
  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.
\$\endgroup\$
  • \$\begingroup\$ "I would even say that it is not as ok as what you did" I dont understand this? \$\endgroup\$ – netizen Jan 10 '17 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 '17 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 '17 at 17:12
1
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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 '17 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\$ – Jack Creasey Jan 11 '17 at 6:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.