Hi,

I'm pretty new to analog electronic. I've been learning from appnotes and datasheets, and i've sometime seen reference of slew rate VS input level.

I've understood the effect of slew rate in audio, and i'm sure it's a really important parameter as it can act like a distording filter for higher frequencies if its too low. But in most of datasheets, there is nothing about that. So, i'll try to explain what i've understood.

For op amp being stable at unity gain, they got (often/alway?) a internal compensation capacitor wich can be charged from power supply, or bias current, depending on designs. I've seen sometime graphs that show slew rate is lower for lower inputs. Slew rate being maximal at the max input range.

So, using a low power opamp for portable audio, slew rate depend more of input voltage. I need/want a low power op amp for low gain (1-18). But input varies is bellow 3Vpp. So, if slew rate varie too, low level signal can be "slewing" at higher frequencies.

The ad8641 interest me a lot, because i need high impedance (> 5M) input with low power (so one op amp single ended), so low bias current to reduce noise of the resistor going to ground, and that can be power supplied at +/-9v simply with 2 9v batteries giving a true ground. As i need 5 stages, it would be long lasting enough with the ad8641.

But from what i've understand, i cannot be sure to not be "slew-rate" limited since there is nothing on the datasheet about this.

From what i was able to understand, does it means that the ad8641 (powered at +/-13v) have a 3v/us slew rate at output ONLY near it maximum input voltage range (around 20Vpp) ?

What kind of slew rate can be expected of the ad8641 below 3Vpp ?

From what i've seen on some forum, I suspect that this slew rate limiting/filtering can be pleasant and volontary used in some audio effecy designs.

Thanks

Hi Noury,

The slew rate of AD8641 is specified at typical value of 3 V/us at +/- 13 V supply voltage. You should expect this value of slew rate even if your input is below 3 Vpp. Slew rate of an amplifier is define as the maximum rate of change of voltage at its output. It also tells us how high in frequency the amplifier can go for a certain voltage level. So if you want to know the minimum slew rate that you need, you can refer to the equation below:

Slew rate = 2 x π x Frequency x Peak Voltage

From the equation, you need to fill in your maximum frequency and your maximum output voltage (Vpeak) to get the minimum slew rate of the amplifier. This equation will help you in choosing amplifiers with enough slew rate for your applications.

Another equation is for Full power Bandwidth:

Full Power Bandwidth = (Slew rate) / (2π x Peak Voltage)

This equation allows us to define the maximum frequency at which the slew limiting doesn't occur for rated voltage output.

I hope this helps. And let us know if you have any other concerns.

Best regards,

Emman