Lets talk about Slew rate of an Opamp - Page 1
Forum section: electronics / beginners

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 15, 2018, 11:51:20 pm
content: As i was getting closer to buying an opamp, i wish to clear somethings about slew rate of an Opamp.Forgive me if i've not explained my idea but please ask if i need to be more specific about anything. I have a spot welder with which i build packs of 18650 i wanted to add an opamp to it as peak detect and sample the voltage and send it into an arduino. My current schematic is attached with an LT1078 opamp (http://cds.linear.com/docs/en/datasheet/10789fe.pdf). This will be used as precision rectifier and as a buffer. How do i know if the selected opamp would work based on its slew rate. i've also attached some readings from my oscilloscope first one at 40ms and second at 100ms. They probed on two points on the secondary of the MOT, approximately 5cm apart.Would appreciate if anybody could advise.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 16, 2018, 02:54:50 am
content: Datasheet says about 70mV/us and a SS GBW of 200KHz. Not surprising for a micropower opamp. Too slow for your application (as best I can divine from your description).

author: w2aew
author_rank: Super Contributor
timestamp: February 16, 2018, 03:39:13 am
content: I have a video on op amp gain*bandwidth product and slew rate:

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 16, 2018, 06:17:05 am
content: Quote from: Wimberleytech on February 16, 2018, 02:54:50 amSo would it be correct to say that the slew rate should be higher than the one required, same with frequency as well. If that is so then how high can it be ?Based on the above screenshot will a 1V/us and GBW of 6Mhz be perfect. Secondly will this specified opamp work at lower measurements as well ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 16, 2018, 09:01:17 am
content: forgot to add the schematic.

author: Dave
author_rank: Super Contributor
timestamp: February 16, 2018, 10:58:00 am
content: Your schematic is awful. Move things around a little bit, give the components some spacing, don't cross the lines unnecessarily, make it clearer.Not only that, it also has several problems:positive feedback - this thing is going to saturate itself as soon as you turn it on; you can't have gain on the second amplifier like that,overloaded first amplifier, nothing is limiting the current into the cap,there is no way to discharge the capacitor and reset the circuit.

author: Brumby
author_rank: Unknown
timestamp: February 16, 2018, 12:15:13 pm
content: Quote from: anishkgt on February 16, 2018, 06:17:05 amAt least equal to, but greater would be better.QuoteFrequency is part of the deal ... not something separate.I'm no op-amp guru, but my back of the envelope calculations come up with this:For a 1V p-p signal at 6.7MHz (taken from your scope screen shots) ....- for unity gain you will need a slew rate of 21V/us (or better) for no slew rate limiting. If you don't mind a bit of distortion, but will still give you much the same peak value, you could start around 13V/us- if you want a gain of 2, then double those figuresand so on.

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 16, 2018, 12:34:26 pm
content: I guess this way to do measure the voltage would be a bad idea with a variable frequency, correct ? I tried simulating this on LTSpice, for constant frequency this would be ok but when going above 100Khz everything changes and gets distorted.This mV is section of a secondary wire from a MOT. This would be to measure the voltage drop and using ohms law measure the current.

author: Zero999
author_rank: Super Contributor
timestamp: February 16, 2018, 01:28:42 pm
content: Quote from: anishkgt on February 16, 2018, 12:34:26 pmThe circuit most likely didn't work at 100kHz, because the op-amp doesn't have a high enough slew rate.The slew rate is how fast the signal changes, in volts per second. If the signal linearly ramps up from 0V, to 1V, in one second, then the slew rate is 1V/s. If the signal linearly changes from 0V to 1V in half a second, then the slew rate is 2V/s.The slew rate of a sine wave is equal to the following formula:Slew Rate = 2*pi*F*VWhereF = FrequencyV = peak Voltage.See the site linked below:http://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-slew-rate.php

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 16, 2018, 02:24:26 pm
content: Yes I can understand the formula. But after learning more about opamp a I guess measuring the variable voltage and frequency this way with an opamp wouldn’t be the best way to go. Maybe Schottky diode would be a better choice here.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 16, 2018, 03:08:06 pm
content: Why do you title your schematic "Current Sense" when you are doing a peak detect on a voltage?

author: anishkgt
author_rank: Frequent Contributor
timestamp: February 16, 2018, 03:12:12 pm
content: Initially my idea was to use the Peak detect circuit to sense the voltage and use that voltage to scale enough to be fed into an arduino ADC and use that reading to measure the current using ohms law.

author: Wimberleytech
author_rank: Super Contributor
timestamp: February 16, 2018, 03:31:18 pm
content: Quote from: Dave on February 16, 2018, 10:58:00 amAgree on all but one point...on close examination, it appears that the feedback polarity is correct. The second instance is flipped vertically and the polarity symbols are hard to see.

author: rstofer
author_rank: Super Contributor
timestamp: February 16, 2018, 03:38:54 pm
content: When I look at the scope trace and think of the slope of the first fallling edge (chosen because deltaV is a max), I see something like 1V change in 100 ns or 10V change in 1 us. Any way you express it, it is simply a measure of the slope of the voltage traceYour proposed op amp has a slew rate of 0.07V/us which isn't even close! You need one about 150 times faster.There are some very fast op amps like 600V/us for the Linear LT1357. I'm not suggesting that op amp, I just wanted to know if there were fast op amps around.http://www.linear.com/product/LT1357

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2018, 06:39:04 pm
content: Quote from: Wimberleytech on February 16, 2018, 02:54:50 amThis goes double in an open loop peak detection application where the operational amplifier must recover. There are operational amplifier based closed loop peak detection circuits which are less accurate because they depend on the matching of a pair of diodes but much faster.The schematic is awful just like it was last time. I'm not studying it again.