If the CD output voltage is 2V RMS maximum and if the band is limited by the IV filter at 22 khz,
why I should need a slew rate > 2 V/us in my output opamps (after the DAC)?
Ideally, the circuit's slew rate should be at least 5 times the maximum output signal's slope
for minimum distortion.
How do you explain this? Slew rate is a circuit's ability to draw the function 2*Pi*f*V. For f=20kHz and V=5.6Vpp (2V RMS) the required slew rate is approximately 0.8V. Why do I need 5 times higher slew rate than what is required for non-distorted output of the input waveform?
Slew rate is measured by overloading the amp input stage. It overloads because it's not fast enough to follow the rise of a square wave. So slew rate is the amps recovery time it takes to get back into its linear region not a measure of its bandwidth (speed) That's why the 5 ( I've heard 10 ) times.
If an op amp slew rate is not fast enough for a given input frequency, a rolloff will occur, not overload.
Slewing is rather like clipping, but due to excessive signal slope, rather than excessive amplitude.
During slewing, the signal waveform is replaced by the amplifier's innate limiting output slope.
A sine wave input would indeed measure lower peak amplitude, but would also be altered to have
a sort of sloping "crossover distortion" near the zero crossings, which is what lowers the peak amplitude.
The maximum signal slope of a sine wave, which occurs at the zero crossings, is equal to the
peak amplitude x 2Pi x frequency, since d/dt (A sin(2Pi x f x t)) equals 2Pi x f x A cos(2Pi x f x t).
This has a maximum value of 2Pi x f x A, where t=0 and cos(0) = 1, at the sine's zero crossing.
For example, a CD player's maximum slope output with a 20kHz sine input is around
2V x 1.414 x 2Pi x 20kHz, or 0.36V/uS maximum slope.
So you are saying the difference between these opamps (2134, 827, and 627) is due to the measured differences in settling time rather than slew rate?? The data sheets quote these figures: 2134---20v/uSec slew rate, 1000 nSec settling time; 827--- 28v/uSec and 850 nsec; 627---55v/uSec and 550 nSec.I just went through a huge learning experience with testing different types of op amps and slew rates in a digital line. Once I figured out how slew rate affected everything, all the different opinion threads just went out the window (I tried xxx op amp and it's better than yyy op amp). There are different quality between op amps. I have found that the opa2134 (which is so popular), even though it has a slew rate of 20V/uS, is really not a great op amp. It's muddy sounding with a lot of distortion in the highs. It's also not fast enough to have good separation of instruments and high frequency excitement. A set of dual opa827 op amps (slew of 20V/uS) are much better. It is also cleaner sounding since the settling rate is faster. An extremely fast opa627 (slew of 55V/uS) will attempt to reproduce the wave form as exactly as it was seen on input.
So you are saying the difference between these opamps (2134, 827, and 627) is due to the measured differences in settling time rather than slew rate?? The data sheets quote these figures: 2134---20v/uSec slew rate, 1000 nSec settling time; 827--- 28v/uSec and 850 nsec; 627---55v/uSec and 550 nSec.
I just checked and OPA627s at Mouser are $25 EACH!!!! Anyone know of a cheaper source?