Measuring opamp slew rate

Thread Starter

swr999

Joined Mar 30, 2011
27
I'm wondering, for any "general-purpose" opamp, if I configure it as a non-inverting unity-gain buffer and run it in the following conditions:

circuit: unity gain buffer
signal input: 200 mVpp sinewave
frequency: unity gain GBW frequency

then, with an oscilloscope I would measure the rise time (10-90%) and do the following calculation:

ΔV = (0.2 - .04) = 0.16V (rise time voltage change, subtracting out the initial 10% and final 10%)
RT = measured rise time value from scope (uS)
slew rate = ΔV / RT (volts/uS)

I just want to get in the right ballpark. Does this make sense?

I would have provided a schematic but I didn't see a tool to draw one on AAC.

Thanks all!!
 

OBW0549

Joined Mar 2, 2015
3,454
If you want to test slew rate input a square wave signal, not a sine wave. Then simply view the output rise/fall time directly on your scope and compute the slew rate from that.
 
I usually use a square wave instead of a sinewave as input for slew rate. It is easier to pick the 10% and 90% points.

Since you are using a unity gain buffer and a peak to peak sinewave, I would expect one of the % readings to be negative. With 200 mVpp input your voltages that I would expect your output measurements would be more like DeltaV = V10% - V90% = 0.08V - (-0.08V).
 

Zeeus

Joined Apr 17, 2019
597
I'm wondering, for any "general-purpose" opamp, if I configure it as a non-inverting unity-gain buffer and run it in the following conditions:

circuit: unity gain buffer
signal input: 200 mVpp sinewave
frequency: unity gain GBW frequency

then, with an oscilloscope I would measure the rise time (10-90%) and do the following calculation:

ΔV = (0.2 - .04) = 0.16V (rise time voltage change, subtracting out the initial 10% and final 10%)
RT = measured rise time value from scope (uS)
slew rate = ΔV / RT (volts/uS)

I just want to get in the right ballpark. Does this make sense?

I would have provided a schematic but I didn't see a tool to draw one on AAC.

Thanks all!!
Not sure you are right

V = Vp *sin(wt)

slew rate is |dv/dt| = Vp * w

Your equation should be multiplied by pi/2 I think
 
Last edited:

LvW

Joined Jun 13, 2013
1,056
Slew rate MUST be measured with a square wave input (as mentioned in post#2).
The opamp must have negative feedback - normally, in unity gain configuration.
However, this input signal must not drive the output into saturation.
The 10% and 90% points (important for small-signal bandwidth only) have nothing to do with slew rate which is a large-signal parameter.
Simply measure/simulate the slope d(Vout)/dt.
 

schmitt trigger

Joined Jul 12, 2010
278
The only requirement is that the input signal has a risetime/falltime much faster than that of the opamp's.

The frequency is not relevant here, only to allow a good repetition rate to properly view on a scope.
Although with digital scope, this is no longer a requirement.
 

crutschow

Joined Mar 14, 2008
25,126
Slew-rate can affect the high-frequency sine-wave maximum output amplitude, but slew-rate is measured with a square-wave input.
 

Thread Starter

swr999

Joined Mar 30, 2011
27
It looks like I was over-complicating what should have been a straight-forward measurement. A squarewave input, and using the oscilloscope cursors to measure ΔV/Δt, or a direct scope readout of the 10%-90% risetime, seems to be the way to go. And as pointed out by @schmitt trigger, the squarewave input signal needs a slew rate much faster than that expected for the device under test. In my case I am using an HP33120A, where for a squarewave I measured a slew rate of ~330 V/uS, much faster than the ~1V/uS I measured for my amplifier (MCP6271).

Thanks to all for your comments!!
 
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