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Lm358 output gain instability

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Richard234

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Hi everyone,
please i am having an issue, with my circuit, i built an non inverting amplifier with a a gain of around 100, the input to the op amp is form a shunt whose output is 100mv@10A ,everything works fine when the input voltage is above 50mV, but when my input is about 40mV output gain begins to be unstable, and lower input the lower the output gain.
it got to a point were i had 2.0mV and my output was 10.1mV. And i intend measuring from 0.1mv(10mA) to 200mv(2A). please i would really appreciate if anyone could help me one this...i have been on this for quite some days now
 

Hi,

Without a schematic a detailed discussion is impossible.

But maybe it´s just a VOL problem.

Read datasheet "Electrical Characteristics", paramater "VOL".


Klaus

added:
Maybe VOL combined with negaitve input offset voltage.

If you want to measure down to 0.1mV then you should not choose an OPAMP with +/-1 up to +/- 5mV of offset voltage.
 

Hi everyone,
please i am having an issue, with my circuit, i built an non inverting amplifier with a a gain of around 100, the input to the op amp is form a shunt whose output is 100mv@10A ,everything works fine when the input voltage is above 50mV, but when my input is about 40mV output gain begins to be unstable, and lower input the lower the output gain.
it got to a point were i had 2.0mV and my output was 10.1mV. And i intend measuring from 0.1mv(10mA) to 200mv(2A). please i would really appreciate if anyone could help me one this...i have been on this for quite some days now

Could you provide the circuit diagram atleast???
 

Hi,

Without a schematic a detailed discussion is impossible.

But maybe it´s just a VOL problem.

Read datasheet "Electrical Characteristics", paramater "VOL".


Klaus

added:
Maybe VOL combined with negaitve input offset voltage.

If you want to measure down to 0.1mV then you should not choose an OPAMP with +/-1 up to +/- 5mV of offset voltage.

please what dose "Vol" stands for and what op amp would you recommend which has a low offset voltage
 

Hi,

If you have a negative supply in the circuit you can bias the 358 to eliminate the input offset voltage with a trimpot or resistive divider.

- - - Updated - - -

Or if possible, which no doubt not for one reason or another, use a larger shunt value to make Vos virtually irrelevant in comparison to the shunt voltage at lowest current across it.
 

OP AMP.png
this is the schematics of the circuit

- - - Updated - - -

do you mean a voltage divider at the non inverting pin? because i saw something like that one a blog
 

Hi,

please what dose "Vol" stands for
VOL = Voltage Output Low

You live in a modern world, so use it.
google: https://www.google.de/search?q=opamp+vol
Lot´s of explanations and videos that explain your opamp issue

what op amp would you recommend which has a low offset voltage
There are so many suitable, there are many unknown requirements about your circuit (which you still not shwed), we don`t whether they are available in your country....
Thus go to the internet site of your distributor and use the selection tools.

Klaus

ah - now you uploded a circuit.
No wonder your Opamp do not work ... it has no power supply.
 

LM158 (the whole family, incl LM324 and 2904 as well?)
has a reall lousy output stage with the low-going leg
very current-limited and unable to swing much below a
volt (output stage is NPN current sink w/ PNP follower).
So you must respect the current and voltage limitations
(output common mode voltage, current) in attempting
to close the loop.

You should also determine whether the problem is
really just a poor Vio (design, or sample) as the error
seems to really blow out as you get to minimal input
differential. This particular op amp excels at nothing
(aside maybe from cost).
 

Indeed I think Vol is the problem.
I assume you are sensing the load current through Rs and attempting to amplify it by 100 times.

What is happening is the output of the LM358 cannot go below a minimum output voltage, as much as 20mV according to the data sheet. This equates to about 0.2mV at the input in your design. Your options are to use a negative and positive supply so 0V in can give you 0V out without it limiting as it goes as little positive as it can manage, or, to use a rail-to-rail op-amp that includes ground in it's output range. Additionally, the inputs of that amp are not well balanced and they can have a large offset on them. You can think of this as magical voltage that appears out of nowhere at the input and gets added to what you think is there. The LM358 design is, at a guess 40 years old and nowhere near as good as modern devices.

Brian.
 

The 20 mV VOL is for sinking current. Output voltage can be lower with respective load resistor to ground, and LM358 can show a linear input to output relation down to below 1 mV output voltage. I'm not sure if it works with 100k load, better add a low kohm load resistor.

The large LM358 offset voltage already mentioned in post may however cut input voltages below a few mV. You can compensate the offset voltage or add a dedicated positive offset to the measured signal to shift it into the observable range under all circumstances, the offset voltage drift will still make your low voltage measurements unreliable. That's why you want a precision amplifier for this application.
 

LM358 can indeed go to zero Vout under the right conditions - it is one of the few older types that can.

If the load is 100 ohm to gnd it will perform very well - as long as you have nulled the Vios.
 

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