Calling OP-AMP Experts!

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#1
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NOOB WARNING!!!

Okay, please forgive me, as I've NEVER used an op amp before. All I know about electronics, I've learned from reading the internet. I've had NO college training, unless you count Algebra 121 and English 121.

That said, I picked up a LM741CN 8-DIP op-amp from RadioShack to see if I could use it as an output buffer for PWM, to do some simple DAC.

I have it set up in a voltage follower configuration, as below:

V+: 5.5V
V-: GND
Non-invert input: 0-5V PWM from dual-stage RC filter
Invert input: Jumped to output
Offset null pins: NC

I can get the output to swing to around +5.43V just fine. The problem is as soon as the input goes lower than ~1.92V, the output freezes and will not drop below that value, even if I put a resistive load across the output.

Is this just a product of the LM741, or have I done something incorrectly?

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Well, part of your problem is that an LM741 OP-Amp was designed to operated off of a +/- volt supply, such as +/-15VDC.

Radio Shack is way behind the time in everything but cell phones! And even then, I think that are performing a dis-service to the public by selling their junk.

What you want is an OP-Amp that will run off of a single supply and have the property that is called "Rail-to-Rail " output. But even then, you will never totally reach VCC or zero volts output because of internal properties of the output stage of the amplifier.

Part numbers elude me at the moment but, you can go to the Digi-Key website at: http://www.Digi-Key.com and look at a selection of about 200 or 300 OP-Amps - about half of which will be single supply, "Rail-to-Rail " operation.

I hope this helps...

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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If you want audio out then there are chips to simplify this, such as LM386
http://web.mit.edu/6.115/www/datasheets/LM386.pdf

For more reading on op amps, then Walter Jung wrote some good books a while back, e.g.
http://books.google.ca/books?as_auth=Walter+G+Jung&ots=rdhHBmkz06&sa=X&oi=print&ct=title&cad=author-navigational

Good Luck.

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The LM324 will swing down to ground if that is more important for your pwm. Actually a comparator might be better for this.

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The most popular "rail to rail" opamp is LM358 (LM358 has 0 - (VSS-2) swing, if you want output up to 5V the VSS must be >7V)
http://www.national.com/mpf/LM/L...
Another "rail to rail" opamp with very good properties is MCP60x (MCP60x is almost real rail-to-rail from both sides)
http://ww1.microchip.com/downloa...

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The suggestion above about using a comparator is NOT a good suggestion.

In this application, rail rail INPUT and rail-rail OUTPUT are important. Otherwise, you will get distortion. And, because the input and output are tied together, you need the input to be able to swing as far as the output does.

'741 was made before rail-rail was even an idea. It was really built to run between +15V and -15V supplies. And, it will not go much better than 2-3V from either rail.

Don't have any good suggestions for a part. There are many out there. Some are less expensive than others. Some are low power supply current. Sone are high bandwidth. Some are low noise. I have used LMV931 with some success; its in a SOT23-6 smt package and not hard to solder.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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One possibe rail to rail type, that is avalable in DIP8 is TS912 (dual), so it does not fit the 741 socket.
For a newbie, sockets is a good idea.

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Thank you all for your help! A lot of good responses on this one....

I pretty much figured that what I got at 'the Shack' was junk (at least for my application.) It was worth buying though, if nothing else, just to learn.

What's funny, is I went to bed, and as soon as my head hit the pillow, I remembered seeing "rail-to-rail" as a feature filter on Digi-Key, and I thought to myself "I bet that's what I need...."

At a quick glance, it looks like the BUF04701AIDGSR from TI might be a good choice for me... rail-to-rail input/output, and advertised to swing within 50mV of both rails (with a low load - guess I'll have to see what real-world results looks like).

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Couple of quick comments:

1. CMOS op-amps will get you closer to the rails. I've used LM358 quite a bit. They're good for some things, but rail-to-rail ain't one of them. A pin-for-pin CMOS replacement for the LM358 is the TL082 (digikey 296-1781-5).
2. If you can't quite get to the bottom rail, check out the MAX660 data sheet. This takes a 5V input and give -5V out. Use the -5V as the negative supply.

Regards,
Paul

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pkafig wrote:
...check out the MAX660 data sheet. This takes a 5V input and give -5V out. Use the -5V as the negative supply.

And, the MAX690 is a +5VDC to +/-10VDC converter. My understanding is that it is the same circuitry as that used in the MAX232 RS-232 level converter IC. I have used this converter in a couple of projects and it does quite well. It is limited on output current, though.

And to that same thought, in a pinch, you could just use a MAX232 as, it will also output +/-10VDC.

You can avoid reality, for a while.  But you can't avoid the consequences of reality! - C.W. Livingston

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GOTO
national.com
-> aplication news, download AN29 and AN31, print, and use it :-)

HM

HM

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Quote:

And to that same thought, in a pinch, you could just use a MAX232 as, it will also output +/-10VDC.

Carl, that's a really interesting idea. I use the MAX232 on nearly all my boards. Where would I take the outputs?

Thanks,
Paul

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741 opamps are certainly the most general purpose opamp, so it makes sense for the Shack to stock those, but they arent very good for audio, because the output wont swing +- volts past about 1KHz, and they are noisy compared to newer opamps, so they are good for things like a voltage follower buffer like you have drawn... BUT you said you wanted a pwm buffer, so I think you should have the avr pwm out drive a logic level n channel mosfet... you'll get a good square pwm out of that, and then you filter it with an RC f=1/(2piRC) and run that thru the voltage follower into your line in for audio

Imagecraft compiler user

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pkafig wrote:

Quote:
Where would I take the outputs?
:?:
Paul .... in the datasheet: pin 2 for +10V and pin 6 for -10V.

Like Carl said: very handy .... I use it for the negative bias of an LCD :)

But be gentle on current-draw: less than 5 mA is fine

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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FWIW, the +/- 10V are typically only achieved (or better, approached) with the original MAX202/232 etc. chips: datasheet specifies a *typical* +/- 9V swing into a 3KOhm load to ground which ties with my experience. I've observed typical swings of +/- 5 to 6 volts on competitioner's (cheaper) products.

Andreas

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Alenze wrote:

Quote:
I've observed typical swings of +/- 5 to 6 volts on competitioner's (cheaper) products.
:) . Not necessarily cheaper.
The ST3232SN (from ST) is a low power version: very smart thing. If there is just a slight load on the outputs, it skips the 2nd voltage-multiplier and thus reduces current consumption. When the load increases, and the single stage can no longer meet the voltage-levels as defined for RS232, the second stage kicks in.

It bit me in the nose as I was unaware of this feature. I needed the negative voltage for the LCD (as mentioned): and it didn't work :( Connected the terminal, and the LCD worked :)

Cheers

Nard

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tricia, and Ulyana. You can find them https://www.linuxmint.com/

Dragon broken ? http://aplomb.nl/TechStuff/Dragon/Dragon.html for how-to-fix tips

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Quote:
ST3232SN (from ST) is a low power version: very smart thing. If there is just a slight load on the outputs, it skips the 2nd voltage-multiplier

Good info - thanks, Nard!

Andreas

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For OpAmp circuits, I highly reccomend using the free spice simulatior from Linear Devices.
http://ltspice.linear.com/softwa...

You don't have to be an expert to use it.
It has a simple, Graphical schematic input,
OK Help. Best of all
it has good Accurate OpAmp models, of "Real" parts that you can actually buy.

Stan Schekall sschekall@eaglabs.com

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Um, Stan

This Thread is about 5 years old...

Hopefully the OP isn't still working on this particular project.

That said, your link may still be useful to others in this day-and-age, but don't expect the OP to reply!

JC

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Sorry!
It's my first time. It's nice to feel so welcome.
In the futute I'll try to remember you polite example
and aspire to emulate as much as possible.

I humbly retract my technical advice, as it is so out of date.

nice going DocJC

Stan Schekall sschekall@eaglabs.com

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Hi Stan. Is the rumor true that everyone in Santa Clara works at a semiconductor outfit or a software outfit?

Imagecraft compiler user

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Personally, I do work for a company that specializes in chip testing hardware and software.
But this is not true everyone in Santa Clara.

Why do you ask?

Stan Schekall sschekall@eaglabs.com

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I've always lived in Florida and California seems like a mystical and magical far away place where integrated circuits and extreme sports are invented.

Imagecraft compiler user

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After 18 (or so) years as Chip Designer in Silicon Valley, I can report that IC design as a Full Contact Exterme Sport.

Stan Schekall sschekall@eaglabs.com

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Good. There is a lot of rumor and speculation here about yield and trimming. Some folks think there is one AVR die per family and they can test the ram pages and flash pages and use the ones that are good. Good theory or bunko?

Imagecraft compiler user

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This is a Fundimental part of Chip Design.
All dies are tested and only the good ones are packaged.

Durring pre-package die testing, Analog components such as resistors and OpAmps can be Laser Trimmed,
to meet Spec.

Flash pages must be 100% passing before packaging.

Because Flash Yealds are often very low, many design contain many more Flash pages than is required.
Then durring pre-package testing the working pages are selected and "bonded out". Thus only working pages are are used.
Thus improves yeald.

Stan Schekall sschekall@eaglabs.com

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Nice to have insider info on this procedure.

Imagecraft compiler user

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Very Interesting

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