Measuring slew rate - Page 1
Forum section: electronics / general

author: sahko123
author_rank: Frequent Contributor
timestamp: June 18, 2020, 01:10:07 am
content: I've recently made some discrete opamps and want to chararcterise them all I know about slew rate is that it's is measured in V/us. I assume that it can be measured by adding a 10vpp square wave with a high rise and fall time and seeing how much the amplifier can amplify in the 1us. Say if an opamp can do 10V in 0.1us would it be alright to say 100V/us (if it was capable of such an output theoretically). And would the methodology I suggested be a good way of measuring opamp or other amp slew rate?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 18, 2020, 06:44:32 am
content: The general idea is correct: apply a large signal an look for the rate of change at the output. As one does not care about the exact input voltage and wants to watch the steady slope, there is no need to have a fast rising square wave.Not all OPs like 10 V signal at the input, so that suitable signal depends on the OP. One way want to limit the input current too, not to violate the input current limit. BJT based OPs usually reach there maximum slew rate already with some 100 mV. FET based OP may need 1 V and sometimes even more.

author: tggzzz
author_rank: Super Contributor
timestamp: June 18, 2020, 07:05:20 am
content: The slew rate is, in most cases, due to an internal limited current charging an internal capacitor. That causes a constant voltage rise across the capacitor, which is amplified and appears at the output. Since your opamp is discrete, you will have access to all internal nodes and so could observe it directly.To cause the output to be slew-rate limited, all you have to do is change your circuit's input so that Vin+-Vin-cannot be kept to zero by feedback. Obviously the input stimulus needed to do that depends on the overall circuit, including feedback if any.So, start by ensuring the opamp is operating in a linear region where Vout~=0, then add a very small input and increase the amplitude until you see a slew rate limit. That is most easily observed with a square wave input, but can be seen with sine wave inputs. For bonus points, see if the slew rate limit depends on the common mode input voltage.

author: magic
author_rank: Super Contributor
timestamp: June 18, 2020, 10:54:05 am
content: Quote from: sahko123 on June 18, 2020, 01:10:07 amYes, but mind what Kleinstein said. If you apply 10V to a BJT long tail pair, high current may flow through the input transistor's BE diode and then avalanche the BE junction of the feedback transistor and flow into the output of the amplifier. If you add protection diodes (see NE5532 if in doubt) to save the input stage from avalanche, the signal will still bypass the opamp and drive the output, causing weird artifacts depending on source impedance of the generator.Quote from: sahko123 on June 18, 2020, 01:10:07 amYes. You don't need to drive for one hour to reach 100km/h

author: David Hess
author_rank: Super Contributor
timestamp: June 18, 2020, 04:05:29 pm
content: If you saturate the output by applying a signal to the input without negative feedback, then the results will be invalid because they include recovery time.I think I would apply a sine wave to a low gain inverting configuration and measure when the inverting input starts to deviate from ground. Then the maximum slew rate can be calculated from the output amplitude (gain * input level) and frequency. A triangle wave could also be used.Alternatively use the same configuration with an input square wave which does *not* saturate the output and directly measure the output slew rate with a DSO.

author: TimFox
author_rank: Super Contributor
timestamp: June 18, 2020, 07:38:04 pm
content: When measuring the slew rate of an actual amplifier (op amp and feedback network) driven by a fast square wave or pulse, be careful to tell the difference between the bandwidth limitation (which is a linear effect independent of amplitude) and the slew rate (non-linear effect that depends directly on amplitude). When in doubt, adjust the input magnitude and see if the rise time or slope (V/us) changes.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 02, 2020, 06:39:56 pm
content: A simple way to measure is to build an oscillator out of your opamp:Output to resistor, to inverting input and with a capacitor to GND.Non-inverting input biased at half of Vcc, and then with a resistor also to the output to create a schmitt trigger at 1/3 and 2/3Then measure the rate of change on an oscilloscope.The good old LM741 is an excellent opamp to get this to oscillate with a triangular waveform on it's output.You can measure some known opamps and compare it with slew rate numbers of their datasheets to get some confidence in your measurements.Some opamps do not tolerate high differental input voltages, and "high" is here > 0.6V.For more accurate measurements, do not start measuring from the start of change, but select a fairly straight part of a flank, and zoom in with your oscilloscope on that.