Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Lm358 output gain instability

Richard234

Newbie level 3
Joined
Oct 17, 2018
Messages
4
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Location
Nigeria
Activity points
30
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
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
17,260
Helped
3,906
Reputation
7,810
Reaction score
3,777
Trophy points
113
Activity points
114,604
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.
 

biswaIITH

Full Member level 3
Joined
Aug 23, 2014
Messages
150
Helped
15
Reputation
30
Reaction score
12
Trophy points
18
Location
Hyderabad
Activity points
1,166
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???
 

Richard234

Newbie level 3
Joined
Oct 17, 2018
Messages
4
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Location
Nigeria
Activity points
30
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
 

d123

Advanced Member level 5
Joined
Jun 7, 2015
Messages
1,947
Helped
442
Reputation
884
Reaction score
427
Trophy points
83
Activity points
19,993
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.
 

Richard234

Newbie level 3
Joined
Oct 17, 2018
Messages
4
Helped
0
Reputation
0
Reaction score
0
Trophy points
1
Location
Nigeria
Activity points
30
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
 

KlausST

Super Moderator
Staff member
Joined
Apr 17, 2014
Messages
17,260
Helped
3,906
Reputation
7,810
Reaction score
3,777
Trophy points
113
Activity points
114,604
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.
 

dick_freebird

Advanced Member level 5
Joined
Mar 4, 2008
Messages
6,714
Helped
1,980
Reputation
3,964
Reaction score
1,798
Trophy points
1,393
Location
USA
Activity points
53,839
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).
 

betwixt

Super Moderator
Staff member
Joined
Jul 4, 2009
Messages
14,108
Helped
4,674
Reputation
9,362
Reaction score
4,414
Trophy points
1,393
Location
Aberdyfi, West Wales, UK
Activity points
120,567
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.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
46,732
Helped
13,872
Reputation
27,992
Reaction score
12,516
Trophy points
1,393
Location
Bochum, Germany
Activity points
272,762
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.
 

Easy peasy

Advanced Member level 5
Joined
Aug 15, 2015
Messages
2,386
Helped
884
Reputation
1,768
Reaction score
870
Trophy points
113
Location
Melbourne
Activity points
13,592
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.
 

Toggle Sidebar

Part and Inventory Search


Welcome to EDABoard.com

Sponsor

Sponsor

Design Fast


×
Top