My non-inverting amplifier doesn't work correctly (the Gain isn't correct) [ICL7650S Op-Amp]

Go To Last Post
13 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I have asked my question here.

Why doesn't it work correctly?

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Well.

I do not see at it is a single power type.

all data are notice with +- 5Vdc . no from 0 to 5V...

Thierry

 

 

Thierry Pottier

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 1

It would work if instead of taking the -V power input of the op-amp to ground, you took it to a negative voltage. Alternatively, you could bias the inputs to half way between the V+ supply and ground. For best results you should create that bias with another op-amp, as shown, otherwise the current from the output will move it.

 

In my drawing I added the resistors R1 and R2. R2 is just there to provide bias to the + input and should be much greater than R1. I used the same values as R3/R4 to satisfy the condition that R1 parallel R2 should be the same as R3 parallel R4, for minimum offset. Note that your ground reference point, for both input and output, is the bias point, the output of the second op-amp. If you don't like that, you'll just have to AC couple the input and output with capacitors. On the whole, though, it would be a much better idea just to keep your original circuit and supply the op amp with -5V. When I have a situation like this I take the +V and -V supplies from a MAX232, if I have one on the board, and if I don't I find an excuse to put one on.

 

 

 

 

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks Peter.

That idea about MAX232 is nice and interesting.

But sounds like my problem with Op-Amp is going to be continue! for example again:

 

http://electronics.stackexchange.com/questions/168787/how-to-operate-op-ampop07cp-with-0-5v-supply

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

Last Edited: Sun. May 3, 2015 - 05:17 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Rohalamin wrote:

Thanks Peter.

That idea about MAX232 is nice and interesting.

But sounds like my problem with Op-Amp is going to be continue! for example again:

 

Ok, looking at your circuit, you have a gain of -346. Does that mean the output is going to be -346 times the -ve input? Hell no. It's going to be -346 times the difference between the positive and negative inputs. And you have a bias on the +ve input, which can only be positive. So 0V on the -ve input is always going to be a negative voltage output, because the difference between the + and - is positive, and the output won't go positive until the difference is negative - the voltage on the -ve input is higher than the bias on the +ve.

 

Next question I'm going to ask is, why the hell are you using an OP07? It's a fine op-amp, totally leading edge in, oh, about 1978. Modern op-amps have got so good these days that you almost never need a null offset, and if you do, you better have a finished PCB design with careful layout or it will do you no good. The OP07 can't put its output closer than about 1V to the supply voltages, and if you read its data sheet, you will see its minimum supply voltage is 6V (plus and minus 3V). If you want to run from 5V, you need a low voltage op-amp with rail to rail outputs and input common mode that includes both supplies, such as this one - http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf. (Hey, they may not be able to design a processor but they do ok with other things.)

 

When you run from a single supply, you must bias both the +ve and -ve inputs above ground and below Vcc, and remember, the output is based on the difference between them. So if your +ve input is biased to 2.5V, then your 0V reference input level is also 2.5V.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I changed my Op-Amp to ICL7650S, again.wink

For my job, an MCP6V07T-E/SN is a great option but unfortunately the problem is that that's not available in my country and if I would order it from china, that would be expensive for me.

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

It was just an example. It's also in an inconveniently small package. But why the ICL7650? That's a chopper stabilized op amp. It's very uncommon that you would ever actually need that level of drift compensation - I did once, but that was in 1985 - and it needs very careful PCB layout to take advantage of it. Also, the 7650 will not work on 5V. Its minimum supply is +/- 3V.

 

I guess you're just using what you have available, but from where you are with this design, judging from your questions, any old op-amp would do to get it working. You could worry about esoterica like offset nulling later when it produces output voltages at least in the ballpark of what you expect.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

peret wrote:

I guess you're just using what you have available...

available AND the best for my job.

 

After working for about 2 weeks, I found out that working with it is not easy. probably I will finish my project with an LM358.

If I was in Tehran I could buy an AD8572ARZ but it's not easy for me that live in a small town in Bushehr.

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

Last Edited: Thu. May 7, 2015 - 06:01 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I understand there is much going on in the world today but please let us keep the hatred off of our streets here at AVR FREAKS.

It is understandable to be fustrated at lack of availability of parts for a project but please lets keep the gasoline away from the newspapers.

There is so much of that already. My own opinion only. 

Its really a great bunch of people here who are quite willing to help.

Peace and Kindest Regards also best of luck in getting your circuitry/project to work.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

voyetra_8,

 

I responded to your report. As I couldn't split the post above (bugs in the forum software) I have simply removed the offending text from it and made a copy in the admin forum.

 

I agree with you but please don't let this get out of hand - I can only assume Rohalamin was having a very bad day to post something like that!

 

Cliff (moderator).

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'm just worrying that what would Peter think about my post when he will see it.

maybe he will think I have insulted to him or I have said somethings that was not nice to him or I'm rude.

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!

Last Edited: Fri. May 8, 2015 - 05:42 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 1

Rohalamin wrote:

I'm just worrying that what would Peter think about my post when he will see it.

maybe he will think I have insulted to him or I have said somethings that was not nice to him or I'm rude.

LOL, your post was removed before I saw it, but I know where you live and I know you were talking about procurement problems, so it isn't hard to guess what you said. I am sure you didn't insult me. Peace.

 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

peret wrote:

LOL, your post was removed before I saw it, but I know where you live and I know you were talking about procurement problems, so it isn't hard to guess what you said. I am sure you didn't insult me. Peace.

Sometimes I freak out because of my situation and curse my country.

wink

"One's value is inherent; money is not inherent"

 

Chuck, you are in my heart!