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Problem with unstable gain of LM358 opamp

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Hammer111

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Hi

I have a problem getting opamp to work as I want. What I want is that input voltage of 0 to 20 mV is transferred to 0 to 2 V but when input difference is 8.3 mV I get output of 640 mV (gain is 640/8.3= around 77) but not around 100 :(
I use LM358.
Another test is while input 5 mV output is 300 mV (gain =60)

INPUT IS USED FROM THERMOCOUPLE from soldering hand

I must have gain of 100 all the time

Supply voltage is single (+5V and GND)
 

IanP

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lm358 rail to rail

The LM358 is not the way to go ..
You will always have problems with low output voltages ..
For this application you should consider a rail-to-rail (at least output) opamp ..
Examples:
LMV358M, TLV2252, ..

Regards,
IanP
 

Hammer111

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lm358 application

I tried searching it to buy it but nothing in cathalogue of 2 companies for electronic stuff here
Any other example of opamp to use?
Here's the list:


0180032302 /NA LM323K12 12V 3A *!
0180032303 /NA LM323K15 15V 3A *!
0180002201 LM0022CH *!
0180101401 LM1014N2 *!
0180010101 LM101AH *!
0180010802 LM108H *!
0180010901 LM109K *!
0180001002 LM10CLN *!
0180001001 LM10CN8 *!
0180011702 LM117K *!
0180011901 LM119H *!
0180012401 LM124N *!
0181360001 LM13600N *!
0181370001 LM13700N *!
0180013701 LM137K *!
0180139101 LM1391 *!
0180013901 LM139N *!
0180145802 LM1458DP
0180145803 LM1458FP SMD *!
0180149603 LM1496N *!
0180015801 LM158H *!
0180183001 LM1830N *!
0180188101 LM1881 *!
0180189401 LM1894N *!
0180189701 LM1897N *!
0180020101 LM201AN *!
0180021001 LM210H *!
0180021100 LM211 SMD *!
0180021102 LM211DP
0180021701 LM217K *!
0180021801 LM218H *!
0180220201 LM2202N *!
0180022403 LM224N
0180023401 LM234Z3 *!
0180023501 LM235Z *!
0180023902 LM239N *!
0180025803 LM258N PDIP8 41 S 1588 1.20
0180259600 LM2596S 5.0 SMD *! 29.90
0180290101 LM2901N 1.90
0180290201 LM2902N *! 1.40
0180290301 LM2903N PDIP8 41 S 5858
0180290401 LM2904N 1.65
0180290701 LM2907N14 *!
0180291702 LM2917N14 *!
0180291701 LM2917N8 *!
0180029306 LM293N PDIP8 41 S 1592
0180295001 LM2950CZ-5.0 TO92 *!
0180030101 LM301AHC *!
0180030102 LM301AN

0180304602 LM3046N *!
0180030401 LM304H *!
0180030501 LM305H *!
0180030801 LM308H *!
0180030802 LM308N *!
0180030901 LM309K = 7805 TO3 *!
0180031002 LM310N8 = SFC2310 *!
0180031102 LM311JG *!
0180031103 LM311N PDIP8 41 S 2350
0180314601 LM3146E *!
0180031701 LM317K
0180031703 LM317LZ
0180031702 LM317T
0180031803 LM318H *!
0180031801 LM318P
0180031901 LM319N *!
0180032301 LM323K 5V 3A
0180032405 LM324 SMD 41S3175
0180032402 LM324N
0180032501 LM325N *!
0180032901 LM329DZ *!
0180330101 LM3301N *!
0180033102 LM331N *!
0180033401 LM334Z
0180033501 LM335H *!
0180033502 LM335Z
0180033608 LM336M5 SMD *!
0180033602 LM336Z2,5
0180033601 LM336Z5 *!
0180033703 LM337K *!
0180033702 LM337T
0180033801 LM338K
0180033905 LM339AN
0180033903 LM339N DIP14 41 S 3650
0180034601 LM346N *!
0180034801 LM348N *!
0180034901 LM349N *!
0180035001 LM350K *!
0180035002 LM350T
0180035800 LM358D SMD IC 13*!
0180035802 LM358N
0180003502 LM35DZ TO92
0180037601 LM376N *!
0180038001 LM380N14
0180038002 LM380N8 *!
0180038102 LM381N14 *!
0180038201 LM382N *!
0180038501 LM385Z2,5
0180038601 LM386N
0180038701 LM387N *!

I think this would be sufficient there's some more around 30 but I wont paste it
 

Hammer111

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lm358 applications

I found this:

TLC271CN
TLC272CP

Aony of those suits me? and what is the data to search for to know which one to use
 

IanP

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lm358 gain

None of these opamps exhibits feature (rail-to-rail output swing) that is required in your application ..

If you want to operate with single supply (0-5V) you need an opamp with rail-to-rail output swing, or another option is to use the LM358 and supply it of symmetrical power supply, for example (-5V 0V +5V) ..

GOOGLE using the following key-words:
OPAMP + RAIL-TO-RAIL

Regards,
IanP
 

johnyaya

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thermocouple amplifier lm358

Another technique would be to shift the reference (shunt resistor currently connected to common) to 1.0V This would lift the opamp output off the bottom rail. Too much gain and you hit the other rail though.
 

Hammer111

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lm358+thermocouple

Ok I got AD 623AN from a friend.
I used 413 Ohm resistor to set gain 242 and it works good on low signal but I saw it's top output is 1.17 V (I need 5 V top) so when reaching 20 mV input, output is 1.17 V
Ref pin is tied to ground.

Look at this: ( I need output from 0V to 5 V) what should I connect to ref pin in order to get it
 

IanP

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use of lm358

First of all, in the above example the output voltage range should be 1.211V to 3.077V ..

In your case:
(-) input and Vref (it provides a direct means of injecting a precise offset to the output) should be connected to 0V ..
(+) input 0-20mV ..
R(gain) ≈ 401.6Ω ..
The the output range should be 0V to 5V ..

Regards,
IanP
 

Hammer111

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thermocouple with lm358

I tried it and it's again stuck at 1.17 V as maximum, now I will try dual supply ±5V to see if it works but I need single supply :(
 

faraz101

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lm358 application notes

use ad620 instrumentation amplifier.i used it for 10 to 500 gain in different applications.i used it for 1mV to 30mV input voltage.and its working great.
 

Hammer111

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lm358 thermocouple amplifier

Can you exactly tell me how did you connect it? Every pin
I connected it by picture above (measuring thermocouple's voltage) execpt ref was at GND and without capacitor

Added after 2 hours 44 minutes:

I tried setting gain to 2 and I put 1V and GND ( to +IN and -IN ) and I got 2 V output (ref and -In grounded).

Hmmmm it works but not with gain of 1000 for example I it won't go above 1.15 V. It does amplify signal to 1.15 V but when output reaches 1.15 V it stops for some reason?

Added after 2 hours 49 minutes:

Got it solved .... had to apply aditional Vcm voltage :)
 

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