Slew Rate Calculator
Minimum Slew Rate Needed (for Voltage and Frequency Used)
This page contains 3 different calculators. The first calculator, shown above, calculates the minimum slew rate needed in order to output a certain voltage at a certain frequency. The second calculator, below, calculates the maximum frequency that an op amp can output given the amplitude of voltage that will be output. The third calculator, also below, calculates the maximum voltage that an op amp can output given a particular frequency. Both calculators compute these values based on the op amp's slew rate.
We will now discuss the first calculator shown above, the one which calculates the slew rate, based on the slew rate formula of, slew rate= 2πfV.
Let's first discuss what slew rate. This is important to understand the calculator.
The slew rate of an op is the maximum rate of change of output voltage per unit time. It is normally expressed on op amp datasheets as unit volts per microsecond. So it's the amount of voltage an op amp can output each microsecond that passes. So based on this, the slew rate is time dependent. It's the amount of volts that an op amp can output in a microsecond.
Based on the op amp in use, the slew rates can vary widely.
Some op amps are slow, some are medium speed, and others are high-speed op amps.
The slew rate for an operational amplifier will normally always be on the datasheet.
The LM741, a very popular op amp used in hobbyist electronics, has a slew rate of 0.5V/μS. This means it can output 0.5V in a microsecond.
The 5532 op amp made by Texas Instruments can output 9V/μS.
The LH0063C op amp has a slew rate of 6000V/μS. This is very, very fast and can deal with large voltages at MHz applications.
Even though the datasheet is normally expressed as volts per unit time that passes, the more practical way of seeing it is the amount of volts that an op amp can output for a given frequency. This is because op amps normally deal with AC signals. The signal has a frequency, for example, 10KHz. The frequency of the signal really means the amount of the amount of cycles that pass per unit time. So the amount of time that passes by is directly related to the frequency of the signal. It's the inverse. The frequency is the inverse of the time period of the a cycle of an AC signal. Therefore, we can directly relate the slew rate to the frequency of the AC signal.
So the first calculator calculates the minimum slew rate that is necessary to output a certain voltage at a certain frequency.
Say, we want to output 10V at 25MHz. Plugging this into the formula, slew rate=2πfV= 2(3.14)(25,000,000Hz)(10V)= 1,570,000,000V/s. Converting this to V/μS, we divide this number by 1,000,000 to get 1570V/μS, which is the standard way of showing the slew rate. So the minimum slew rate to output this voltage at this frequency is 1570/μS. So you would have to find a high speed op amp with this slew rate characteristic.
So this first calculator really helps to guide you to an op amp that can meet a slew rate which can satisfy the voltage output you need at a certain frequency.
Maximum Frequency The Op Amp Can Handle (for a given voltage and the op amp's slew rate)
The second calculator calculates the maximum frequency that an op amp can output given a certain voltage value.
Say you want to know the maximum frequency that can be used at a certain amplitude of voltage. This calculator calculates this maximum frequency for this voltage value.
The slew rate on datasheets is defined as volts per unit microseconds. This corresponds to megahertz frequencies, since the inverse of microseconds is megahertz. In order to be on the more level playing field of hertz, we can convert the slew rate from V/μS into V/S by multiplying V/μS by 1,000,000; this gives us V/S. This now allows us to deal with the frequency of hertz instead of megahertz. As we said previously, the LM741 can pass 0.5V/μS. Multiplying this by 1,000,000 gives us 500,000V/S. Using the formula, slew rate=2πfV=2(3.14)(f)(0.5V), the LM741 can output 0.5V at a frequency of 159,235Hz. The LM741 can output 1V at a frequency of 79,617Hz.
So by converting from the time from μS to S, we can get a measurement of Hz from the slew rate formula.
This calculator allows you to see the maximum frequency the op amp can deal with a certain voltage value.
Maximum Voltage the Op Amp Can Output (for a given frequency and the op amp's slew rate)
The third slew rate calculator allows us to calculate the maximum voltage that an op can output at a certain frequency.
The user just has to plug in the slew rate of the op amp and the frequency that s/he wants to find the maximum voltage output for, and the calculator computes this maximum voltage.
It comes in hand if you're designing a circuit and you want to see the maximum voltage that the op amp can output at a given frequency without producing distortion.
The calculator, like all the others, works based on the slew rate formula, slew rate=2πfV.
Let's use the example again of an LM741 op amp, which has a slew rate of 0.5V/μS.
Let's say we want to calculate what's the maximum voltage the op amp can output for a frequency of 30KHz.
First, we convert the slew rate from V/μS to V/S by multiplying it by 1,000,000. This gives us a slew rate of 500,000V/μS.
Then we plug all of this into the slew rate formula, slew rate=2πfV= 500,000V/μS= 2(3.14)(30,000Hz)V= 2.65V. So the maximum voltage that the LM741 can output at 30KHz is 2.65V.
The maximum voltage that the LM741 can output at 10KHz is 7.96V.
So you can see based on these calculations that the LM741, with its slew rate, is really meant to deal with frequencies on the low range of KHz to Hz. It isn't a high-speed op amp, so it can only deal with relatively low frequencies.
This calculator helps determine the maximum voltage an op amp should output given a certain frequency.
Keep in mind that for all these calculators, you have to take into account the maximum voltage that the op amp you are using can take. You cannot use a higher voltage than what
the op amp can handle. Therefore, if you use the calculator and getting a higher maximum voltage than what the op amp can deal with, you have to nullify this and only use the maximum voltage
that the op amp can deal with.