OSC and unity gain buffer
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From: eeaj2002 on 9 Apr 2008 20:30 Hi all, I need help with my circuit please. I have a one MHz osc fed into the positive input of the opamp LMC6484 and the negative terminal is connected to the output (unity gain buffer). At the positive input terminal I have the 1MHz. from the oscillator and I should see one MHz at the output as well but I see distorted sine wave signal. If the gain bandwidth of the opamp is 1.5MHz. should not I see one MHz. at the output of the opamp as well. I did run the oscillator at 3.0 Volts and the opamp at 15V. Thanks for you help in advance, John.
From: Joel Koltner on 9 Apr 2008 20:47 This sounds a lot like a homework problem, but I'll help anyway... -- The slew rate of your LMC6484 there is specified at 1.4V/us -- What slew rate is required to run a 1MHz sine wave at 3V? (Hint: It's rather more than 1.4V/us... a 1V sine wave at 1Hz has a maximum slope of 2*pi volts/second...)
From: eeaj2002 on 9 Apr 2008 21:11 On Apr 9, 5:47 pm, "Joel Koltner" <zapwireDASHgro...(a)yahoo.com> wrote: > This sounds a lot like a homework problem, but I'll help anyway... > > -- The slew rate of your LMC6484 there is specified at 1.4V/us > -- What slew rate is required to run a 1MHz sine wave at 3V? (Hint: It's > rather more than 1.4V/us... a 1V sine wave at 1Hz has a maximum slope of 2*pi > volts/second...) Hi Joel, Thank you for your hint but this is not a homework problem. I was just trying to learn about opamp myself. Are you saying that its slew rate must be greater than 1.4V/us? Thanks, John.
From: Phil Allison on 10 Apr 2008 00:30 "eeaj2002" "Joel Koltner" > > -- The slew rate of your LMC6484 there is specified at 1.4V/us > -- What slew rate is required to run a 1MHz sine wave at 3V? (Hint: It's > rather more than 1.4V/us... a 1V sine wave at 1Hz has a maximum slope of > 2*pi > volts/second...) Hi Joel, Thank you for your hint but this is not a homework problem. I was just trying to learn about opamp myself. Are you saying that its slew rate must be greater than 1.4V/us? ** He did. The maximum slew rate of a sine wave occurs around the zero crossings & is given by the formula: SR = 2 x pi x Vpk x F when F is in MHz, then SR is in V / uS. Your voltage follower must have a SR equal to or greater than the signal it is passing. If you simply reduce the level or frequency of the sine wave sufficiently, the output wave will become sine. ...... Phil
From: Tim Wescott on 10 Apr 2008 12:42
eeaj2002 wrote: > On Apr 9, 5:47 pm, "Joel Koltner" <zapwireDASHgro...(a)yahoo.com> wrote: >> This sounds a lot like a homework problem, but I'll help anyway... >> >> -- The slew rate of your LMC6484 there is specified at 1.4V/us >> -- What slew rate is required to run a 1MHz sine wave at 3V? (Hint: It's >> rather more than 1.4V/us... a 1V sine wave at 1Hz has a maximum slope of 2*pi >> volts/second...) > > Hi Joel, > > Thank you for your hint but this is not a homework problem. I was > just trying to learn about opamp myself. > Are you saying that its slew rate must be greater than 1.4V/us? > > Thanks, > John. The slew rate of the amplifier must be greater than the slew rate of the signal coming out -- otherwise you'll have an output that tries to be a sine wave, but has these straight sections where the amplifier can't slew fast enough. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html |