Opamp output fluctuates on powering on - Page 1
Forum section: electronics / beginners

author: raff5184
author_rank: Regular Contributor
timestamp: November 12, 2020, 10:44:35 pm
content: Hi,I am using an OPA197 in simple non-inverting configuration and a gain of 20xI need to power the opamp on and off continuously to save energy.When I power it on, the output has a peak of 1.2V and then goes to 0 again as showed in the attached pic1 and pic2 (2 is zoomed in).1) Is it a normal behavior?2) How can I prevent it or smooth it out? I tried to use a 1uF bypassing capacitor and it helps a bit (pic3) but it's not enough. Other values (singularly or in parallel to the 1uF) are either not enough or cut my output signal

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2020, 12:05:40 am
content: Quote from: raff5184 on November 12, 2020, 10:44:35 pmIt's normal for the output to be unstable for a short length of time after the power is connected. Is it AC or DC coupled? If the circuit is AC coupled, there will also be large spikes, as the coupling capacitors charge up.There isn't much you can do, apart from adding an analogue switch to the output to connect whatever is connected to the op-amp's output to 0V for long enough for the output voltage to stabilise.

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2020, 01:30:16 am
content: Operational amplifiers may behave unpredictably below their minimum supply voltage which is 4.5 volts for the OPA197. That leaves a couple of options:1. Clamp or disconnect the output when the supply voltage is below 4.5 volts. Note that this must be done while leaving the negative feedback in place or it may create the same problem.2. An operational amplifier with external shutdown might do the trick.2. You might try an operational amplifier with a lower minimum supply voltage, or simply a different operational amplifier which behaves better.

author: raff5184
author_rank: Regular Contributor
timestamp: November 20, 2020, 11:03:54 pm
content: Thank you, I switched to an opamp with SHDN and it's working fine.One last thing I need to do is "flip" the logic of the input signal to the SHDN. When PWM is high the SHDN has to be low, or just floating, when PWM is LOW the SHDN has to be connected to the supply or a high signal. Would a p-channel MOSFET be a good/proper solution? I need something very simple and with the least amount of components

author: Zero999
author_rank: Super Contributor
timestamp: November 20, 2020, 11:15:46 pm
content: Quote from: raff5184 on November 20, 2020, 11:03:54 pmYou can't get simpler, than one IC. A simple logic inverter, such as the 74HC1G14 will do.https://assets.nexperia.com/documents/data-sheet/74HC_HCT1G14.pdf

author: raff5184
author_rank: Regular Contributor
timestamp: November 22, 2020, 03:53:34 pm
content: Quote from: Zero999 on November 20, 2020, 11:15:46 pmThanks Zero, the solution works. But another reason to have a device with shutdown is to save energy, and the Schmitt trigger increases the consumption of 2-3mA which is too much and I didn't find something with low current. I need something in the order of microamps.I used low power comparator ICs in and those might work, but besides complicating the circuit a bit, it seems to be an overshoot to me.A simple NC switch to connect the SHDN to V_supply, that opens when PWM is high is also ok. I tried the TS12A4515 but for some reason when PWM is high the shdwn gets conneted to vsupply but the opamp doesn't enter shutdowm modeThanks again

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2020, 08:12:32 pm
content: Quote from: raff5184 on November 22, 2020, 03:53:34 pmHow lower supply current do you require? The 74HC1G14 uses a maximum current of only 20µA, with its input at either +V or 0V and a maximum of 850µA, with a 5V supply and an input of 3.9V. If it's drawing 2 to 3mA, then you have a bad part, or there's something else wrong.https://assets.nexperia.com/documents/data-sheet/74HC_HCT1G14.pdfThere are lower current options, such as the 74LVC1G14, which only uses 10µA.https://www.ti.com/lit/ds/symlink/sn74lvc1g14.pdf

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 23, 2020, 12:11:20 am
content: Quote from: raff5184 on November 22, 2020, 03:53:34 pmIt's hard to do too much better than a single logic gate here. Don't be confused by the "additional current" specification in the logic gate's datasheets. That specification is trying to describe the behavior where CMOS logic gates draw extra current if their input voltage is near midrail. It's basically shoot-through in the gate's input stage. With a clean logic input pulse to the gate, the input voltage races past midrail, so the current draw is large for only a very tiny amount of time, and integrates to very, very little average power consumed per transition. However, if the input frequency is very high, this can still add up to be non-negligible.The Schmitt trigger is worse than a standard gate, because it has additional leakage paths. These paths also keep the gate stable so its inputs can be held near midrail, such as when the input waveform changes very slowly. If you are driving from a digital source, like another gate or a processor GPIO, you can go with a standard inverter ('04) just fine. (In fact, this is usually the better choice: the Schmitt trigger also adds unwanted jitter.) That said, the Schmitt trigger will not be much more power hungry, but if you have a free choice, you might as well pick the better one....Logic gates are optimized to do this sort of job and do it with as little power as possible. If they look worse on paper than the other options, it is probably because their datasheets are being more conservative. Analog switches and comparators aren't often rated to toggle at 100 MHz; logic gates are, and they still meet all of their specs doing it.In terms of logic family, generally AUP leaks less than LVC, which leaks less than HC. So if you are at 3.3V or below, choose AUP; 5V, choose LVC; 6V, choose HC. (I forget where LV falls in there, much less the other, rarer families. Honestly, you basically want the newest one.) Or choose their differently-named equivalents from other vendors.

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2020, 10:37:05 am
content: Quote from: exmadscientist on November 23, 2020, 12:11:20 amI'm not saying you're wrong about the Schmitt trigger using slightly more power, but the power consumption listed on the 74HC1G04 data sheet, is identical to the 74HC1G14, but they probably slap the same number on all of their data sheets for parts with one or two gates.https://assets.nexperia.com/documents/data-sheet/74HC_HCT1G04.pdfQuoteOne thing to note is that faster isn't always better. The higher speed logic families such as AUP and LVC need more attention paid to decoupling and layout. It's probably a good idea to stick with HC, unless very low voltage, or high speed are important.

author: raff5184
author_rank: Regular Contributor
timestamp: November 23, 2020, 01:10:58 pm
content: Quote from: Zero999 on November 22, 2020, 08:12:32 pmIf possible less than 100uA. So from the numbers you reported it should be fine. Even if my supply is 4V, I shouldn't see more than 850uA.The device I used is the SN74AUC1G14https://www.ti.com/lit/ds/symlink/sn74auc1g14.pdfthat has a max Icc of 10uA but it is rated for max 3.6V (maybe I burned it). I'll run more tests an double check my setup and chip.Quote from: exmadscientist on November 23, 2020, 12:11:20 amI don't need speed. My PWM pulse is minimum 30us (33kHz), and voltage between 3.3V and 5V

author: Zero999
author_rank: Super Contributor
timestamp: November 23, 2020, 04:39:39 pm
content: One thing to note is that the absolute maximum rating is of the SN74AUC1G14 3.6V, which means it shouldn't burn up at that voltage, but it doesn't mean it will actually work at that voltage. It's only gauranteed to work from 0.8V to 2.7V. If you've powered it from 4V, then it's quite likely it's damaged and could be why the current draw is so high.There are a few clues on the data sheet which warn that the device will only work from 0.8V to 2.7V.QuoteQuoteQuoteQuoteAs you don't need the speed, I'd strongly recommend the 74HC1G14 or 74HC1G04.

author: raff5184
author_rank: Regular Contributor
timestamp: November 23, 2020, 04:48:33 pm
content: Quote from: Zero999 on November 23, 2020, 04:39:39 pmYep, I'll get those.As for the one I have now, it has very low current (can't even measure it) up to 2.7V, for >2.7V the Icc increases to 2-3mA. Makes sense. Thanks