Siganl amplifying

[hansika]

Hi friends,
i want to design a signal conditioning circuit whose input is 0-5V and output is 6-18V for that i have used LM741 ic and i have done it with that opamp but the problem is it is giving 5mV offset, but i need the output voltage exactly without any offset, can any one please suggest me is there any opamp which will give output exactly(for 0-5V i/p o/p should be 6-18V only).

 

[Nigel Goodwin]

The 741 has null adjustment connections, use them.

Or, for an opamp without internal null adjustment, just add a suitable bias offset to correct it.

 

[hansika]

i have a doubt ,can we give 24V to 741 IC (to pin no 7) -24V to pin no 4 in multisim software it is wortking fine but real time i want to know whether it is working for 24V or not

 

[audioguru]

i have a doubt ,can we give 24V to 741 IC (to pin no 7) -24V to pin no 4 in multisim software it is wortking fine but real time i want to know whether it is working for 24V or not

It looks like you did not look at a datasheet for a 741 opamp.
The datasheet for an LM741C opamp lists an Absolute Maximum supply of plus and minus 18V and all spec's are listed with a plus and minus 15V supply.

Multisim software also did not read the datasheet.

 

[Hero999]

He didn't specify which 741 he's using.

The C is rated to 36V but the normal 741 is rated to 44V.

 

[audioguru]

He didn't specify which 741 he's using.

The C is rated to 36V but the normal 741 is rated to 44V.

He wants to use 48V which is too high for both.

 

[Hero999]

How do you know that?

Can you mid read or did he say in another thread or forum?

EDIT:
Sorry, didn't read the post properly he said +24V to the +V pin and -24V to the -V pin.

 

[hansika]

can we give -22V to opamp, if we do like that is there any performance failure for long term purposes?

 

[audioguru]

can we give -22V to opamp, if we do like that is there any performance failure for long term purposes?

Usually a dual polarity supply has the same voltages. The 741 opamp was designed for a plus and minus 15V supply. If you use plus and minus 22V then a 741C opamp that has a max allowed supply of plus and minus 18V will fail soon.

 

[hansika]

but some op amps can be operated on +/- 22V for that type of op amp can we give this +22V, what will be happen for long term purpose.

 

[audioguru]

can we give this +22V?

Are you also giving the opamp a -22V supply?
why do you need such a high supply voltage for the opamp?

 

[hansika]

i want to give +20V at positive input ang -5V at negative input i want to know the performance of the op amp for longterm applications.

 

[unclejed613]

another thing you need to look at is the max differential voltage of the input stage of the op amp you're using. putting +22V on one input and -5V on the other will destroy most op amps. if that's what you're doing, then you need a different solution, maybe an instrumentation amp using more than one op amp.

 

[hansika]

Hi,
In signal amplifying if we use op amps we will get some offsets means for 0V i need to get 6V but in real time i am getting 6.02V which will be 20mV offset, how we can avoid this, some signal conditioning cards they will give exact outputs how they will achieve this.

 

[Nigel Goodwin]

Hi,
In signal amplifying if we use op amps we will get some offsets means for 0V i need to get 6V but in real time i am getting 6.02V which will be 20mV offset, how we can avoid this, some signal conditioning cards they will give exact outputs how they will achieve this.

Refer to post #2 which has already answered this.

 

[hansika]

k i will do that, but how the signal shifting cards achieve this

 

[Nigel Goodwin]

k i will do that, but how the signal shifting cards achieve this

Just as post #2 describes.

 

[unclejed613]

also, with standard 5% tolerance resistors, getting 20mV error with a 6V output is actually pretty close (less than 1/2%), and not necessarily an offset error but possibly a gain error. an offset error would be 20mV with no signal. so just what are you trying to measure that requires better than 1/2% accuracy?

 

[hansika]

ya what you saying is correct, here i am using variable potentiometers for the resistances so in resistance no tolerance is present, if it is gain error then how can i eliminate that i need exact output with 5 or 6 mV offset only.

 

[unclejed613]

to start with, install the offset trim recommended in the data sheet. second, whatever you use for an input resistor, use one of equal value on the opposite input (this is essential with a 741 which has high input bias currents) this also should be covered in the data sheet. if it isn't, read application note AN-A from National Semiconductor, which will explain it in detail with examples for both inverting and noninverting circuits. to mitigate the gain error, use 1% tolerance resistors, or use added low value series or very high value parallel resistors to trim your feedback resistor. it would help me be more specific as to what you can do to eliminate the errors if i knew what you are trying to accomplish. a schematic would help.

you will find on this forum that vague questions don't often get very good answers. that's because answering vague questions require a "shotgun" approach. all we really know so far is that you're trying to measure or level shift a voltage with a voltage range outside of the abilities of most op amps with a level of precision and/or accuracy that isn't commonly required, and with no clue what you're trying to accomplish as an end result.