How to measure slew rate of Op-Amp Practically?

[SharpSlayer]

Hi, I am under a internship and my supervisor wants me to practically measure the slew of AD8602ARMZ op-amp.

I am aware that this value is available in the datasheet along with a well plotted graph, but my task here is to recreate the graph on the oscilloscope. So far I have read the basics of slew rate and learnt how to calculate it if I get a graph. But I am not sure if I am in the right path and if this will be possible.

My knowledge and idea on Op-amp slew rate and how to measure it come from this article here: https://components101.com/article/u...in-op-amp-and-how-to-measure-and-calculate-it

Here is what I have done so far,

1. As discussed in article, I have created a voltage follower circuit using AD8602ARMZ op-amp.
2. I have verified that the op-amp circuit is working my measuring the input and output voltage
4. Used an Arduino to generate PWM signals for the input signal of voltage follower
5. Mounted one channel of my scope to input and other channel to the output of my voltage follower circuit

With that I am stuck. The input wave from arduino and output wave from op-amp both look identical I am not able to measure any change in time or change in voltage.

I have tried changing the freqency of input pulse but it did not help. would appreciate if anyone can suggest a way forward.

PS: I am not allowed to take pictures inside my campus, so unable to click and share any

 

[KlausST]

Hi,

my supervisor wants me to practically measure the slew of AD8602ARMZ op-amp.

Why .. and how?
Why: just to verify the datasheet specification of the bare OPAMP, or of YOUR_OPAMP_CIRCUIT including your connections, your schematic, your powersupply, your load?

In the first case you need to set up the identical test conditions as in the datasheet.
In the second case you need to be aware, that the resulting value may differ from the datasheet.

Small signal or large signal?

but my task here is to recreate the graph on the oscilloscope.

The AD8602 datasheet tells you how. So why refer to a "second source" document? (I´m not saying it´s wrong, but I like to understand)

Here is what I have done so far,

--> show your complete circuit, including, power supply, scope and connections.
A schematic, a draft, anything is better than nothing. Also: what scope, what setup and what jig...

You know specifiying electronics works with values and units, not with text.
--> so give values for: frequency, voltage levels, input_slew_rate, output levles, output_slew_rate...


Klaus

 

[danadakk]

You need to look at edge of input and output at high sweep rates. Its typ 5V / uS,
so set your sweep rate to ~ 500 nS or so, or used mixed sweep expansion of that
region around the edge. Then measure slope, delta V / delta t (V / uS). If you
have a modern scope it will have a tracking vertical and horizontal measurement
capability that will do this easily.

Mixed sweep rate :

This is a large signal property so feed OpAmp with a few V pk-pk input.

From datasheet :

Other OpAmp measurements :

https://www.analog.com/en/resources/analog-dialogue/articles/simple-op-amp-measurements.html

Regards, Dana.

 

[khizer kashif]

Hi, I am under a internship and my supervisor wants me to practically measure the slew of AD8602ARMZ op-amp.

I am aware that this value is available in the datasheet along with a well plotted graph, but my task here is to recreate the graph on the oscilloscope. So far I have read the basics of slew rate and learnt how to calculate it if I get a graph. But I am not sure if I am in the right path and if this will be possible.

My knowledge and idea on Op-amp slew rate and how to measure it come from this article here: https://components101.com/article/u...in-op-amp-and-how-to-measure-and-calculate-it

Here is what I have done so far,

1. As discussed in article, I have created a voltage follower circuit using AD8602ARMZ op-amp.
2. I have verified that the op-amp circuit is working my measuring the input and output voltage
4. Used an Arduino to generate PWM signals for the input signal of voltage follower
5. Mounted one channel of my scope to input and other channel to the output of my voltage follower circuit

With that I am stuck. The input wave from arduino and output wave from op-amp both look identical I am not able to measure any change in time or change in voltage.

I have tried changing the freqency of input pulse but it did not help. would appreciate if anyone can suggest a way forward.

PS: I am not allowed to take pictures inside my campus, so unable to click and share any

It seems like you've taken some good initial steps in setting up your experiment to measure the slew rate of the AD8602ARMZ op-amp. However, there are a few additional considerations and adjustments you might need to make in order to accurately measure the slew rate.

1. **Signal Frequency**: While using an Arduino to generate PWM signals is a good start, you'll want to ensure that the frequency of the PWM signal is within the bandwidth of the op-amp and oscilloscope. The bandwidth of the op-amp will limit the highest frequency signal it can accurately reproduce. Make sure your PWM frequency is well within this bandwidth.

2. **Input Signal**: The input signal should ideally be a step function or a square wave rather than a PWM signal. A PWM signal might introduce additional complexities due to its varying duty cycle. A step function will give you a clear transition from low to high voltage, making it easier to measure the slew rate.

3. **Amplitude and Offset**: Ensure that the input signal is well within the input voltage range of the op-amp and that the offset is adjusted appropriately so that the signal is centered around ground.

4. **Probe and Grounding**: Make sure you're using proper probing techniques with your oscilloscope probes. Improper grounding or probing can introduce noise or affect signal integrity.

5. **Timebase and Voltage Scale**: Adjust the timebase and voltage scale on your oscilloscope so that you can clearly see the rise or fall time of the output signal.

6. **Triggering**: Use triggering on your oscilloscope to stabilize the display. You can trigger on either the rising or falling edge of the input or output signal, depending on what you're trying to measure.

7. **Calculation**: Once you've captured the input and output signals on your oscilloscope, you can measure the time it takes for the output voltage to change from a certain percentage (e.g., 10%) to another percentage (e.g., 90%) of its final value. The slew rate can then be calculated using the formula:

Slew Rate = (V2 - V1) / (t2 - t1)

Where:
- V2 and V1 are the final and initial voltages, respectively.
- t2 and t1 are the times corresponding to the final and initial voltages, respectively.

By carefully adjusting these parameters and following proper measurement techniques, you should be able to accurately measure the slew rate of the op-amp and recreate the graph on your oscilloscope. If you're still having trouble, don't hesitate to consult with your supervisor or seek additional assistance from someone with experience in electronics testing and measurement.

 

[danadakk]

The OpAmp bandwidth does limit small signal performance of the OpAmp, whereas slew rate
is a limitation caused (typically) by available current in OpAmp stage with compensation
capacitor or the effects of miller C used for compensation.

https://www.d.umn.edu/~htang/ECE5211_doc_files/ECE5211_files/Chapter6_part1.pdf

https://www.ti.com/lit/pdf/sloa332

https://www.ti.com/lit/an/snoa852/snoa852.pdf

A PWM generated signal for measurement, 50% duty cycle, fine for use in measurement.


Regards, Dana.