Problem with getting 0 offset when changing op amp 741 to higher gain

[johnnykhor]

hi all,

Currently i'm using op amp 741, and getting input from a sensor which produces voltage from 5 to 10 mili-volt range (DC).

I understand that practical 741 having some offset. i used a trimmer of 1k ohm, but the output is stable when i did not do amplifying(buffer).

I couldn't get 0 offset when i change to higher gain(100 times).

Can anyone help me up here? any recommendation?

The other question is, can bypass capacitor help me in this case?

thx in advance for replying.

 

[IanP]

Re: op amp 741

LM741 is an old workhorse and it may still be used in the run-of-the-mill applications but nowadays there are so many really good opamps on the market, so don’t look back and try for example the AD620 ..
https://www.analog.com/static/imported-files/Data_Sheets/AD620.pdf

Rgds,
IanP

 

[karesz]

op amp 741

Hi,
I dont believe that your 741 has effects in relations bypass capacitors...
For 741 offset adjusting its suggested a 10KOhm trimmer to use, w. middle point onto minus supply.
In not working situation, and if your circuit is failure less build; your IC is possible dead.
But for 100x amplifying a max 10 mV signal_is maybe better a moderner amp to use, i.e. OP191_has same pinout, ca. 100 time less offset and is rail to rail I/O, OK these isnt your problem (yet), but it is a clearly better OP.
Or OP 177 has a max offset of 20-50uV on the input...
Offset compensating must be independent from amplifying factor_clear; not the voltage values self, the changes of outputs offset signal must be to see.
K.

 

[LvW]

Re: op amp 741

hi all,
Currently i'm using op amp 741, and getting input from a sensor which produces voltage from 5 to 10 mili-volt range (DC).
..............

Johnny (to be spelled like Johnny Cash?),

don't forget that instrumentation amps are the classical solution for sensor signal amplification. Is your sensor on-sided grounded or not?

 

[johnnykhor]

Re: op amp 741

to IanP,

thanks for the recommendation.

1) if i were to change to AD620, do i still need to do the offset voltage adjusting as 741?

2) is the amplifying ratio and connection of resistor similar to 741?

3) which model is more closely to 741? because i saw a few different models.

sorry if i ask stupid questions, i'm just a student, therefore i need more guidance. thank you very much.

Added after 7 minutes:

to Karesz,

thanks for replying.

normally i so the voltage offset adjustment after all the resistors are connected, while the input is connected to the ground.

is my method correct?
or do i need to take out all the resistor when i adjusting the offset voltage?

thanks for the guidance.

Added after 9 minutes:

to LvW,

really?
741 is chosen because i am more familiar with it, and i know how to do amplification using it. as well as recommended by my final year project supervisor.

yes, one of the leg is connected to ground. is it related to the performance of 741?

 

[karesz]

Re: op amp 741

...yes, one of the leg is connected to ground. is it related to the performance of 741?

Sorry, what is hes Nr/name?
Its so to hear as you will tell: "my name is X" :-(...
Of course is not equal, where are the supply & input pins connected!
LM741 has plus & minus, double, to supply, pls REFER for DATA SHEETS!
You have offset in full connected circuit to adjust...
Which model is closely to 741? >> practically all are close or very better, these is a ca. 35 years old IC... For low DC signals are good input designed ICs to select_we proposed some, but your prof has to agree with one....
AD620 is a very good Idea, is explicit for signal processing developed...
You see, if you have a mountain tour, you will not select an old Ford T1 model_it was good, but nowadays are very better_cheaper & needs less power has higher speed...
Otherwise has an OP177/OP191 i.e absolute same pin out, dont need offset adjustment_their dont have offset pins!
https://en.wikipedia.org/wiki/Operational_amplifier
Input offset voltage https://en.wikipedia.org/wiki/Input_offset_voltage
K.

 

[johnnykhor]

Re: op amp 741

to karesz,

i will take a try, if its not over budget. i was given RM500 for entire project, and i left approx. RM250.

i was wondering, if i able to get 0.00mV offset voltage from my multimeter, is that better than other op-amp where i can adjust it? (of course, it is good only in the sense of offset voltage.)

because i glance through the specs. the offset voltage is in the range of 10-50uV which equivalent to 0.01 to 0.05mV. (it meant a lot in my measurement thou)

 

[karesz]

op amp 741

Hali,
for price & data sheet searching I do beginning regularly at these Databank_Addresse:
**broken link removed**
You can see AD620 is for ca $6-7, OP117 for ca $ 2-3 & OP191 for ca $3 to become_I dont no how its in your country, maybe from China/TAiwan are similar prices to have?

You will have NEVER ZERO OFFSET_but enough_negligible_ small ...
Modern, so called zero compensing (eventuell chopping) amplifiers have offsets at some uVs_in extremes max 10-20uVs, these is a sensor amp! Not a 741...
K.