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Topic: Bioplar to unipolar converter w/ opamp and MAX232 (Read 2226 times) previous topic - next topic

liudr

Been thinking about interfacing arduino's unipolar ADC with bipolar to unipolar converter with an opamp. (I've done the wrong way, feeding ADC with negative vlotages.)

In order to power the opamp, I need positive and negative voltages. Can I use the output of a MAX232 chip (+-12V) to power an opamp? The bipolar signals should be around say -5V to +5V.
Since I'm thinking about real serial port so MAX232 is a must.
Any suggestions? Thanks.

deSilva

#1
Aug 21, 2010, 11:56 am Last Edit: Aug 21, 2010, 11:59 am by mpeuser Reason: 1
The data sheet says max 10mA per line, but also 10 mA max for the whole  chip -  I think one can stretch that a little bit... Note that the nominal output of +/- 15 volts will drop when loaded with < 2k ..

Modern Low Power opamps need around 1 mA + load. And mind that Low Power opamps have no longer necessarily  such a high supply voltage range

RuggedCircuits

You can also use a dedicated charge pump chip like the TC7660. You can double +5V to +10V then invert +10V to -10V. The MAX232 basically has two of these inside it but perhaps you want more control.

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Check out our new shield: http://www.ruggedcircuits.com/html/gadget_shield.html

deSilva

I have forgotten to mention: A most popular unexpensive Low Power Op Amp ist LM358 (Two in a 8 pin DIP for around 20 cents)

liudr

Thanks deSilva. I have a few 741s in my drawer.There's not much "LOAD" since after the convertion and amplification (shifting), the signal is fed to arduino's ADC, which should take very little current, right?

RuggedCircuits, thanks. That chip is inverting the input to the exact opposite right?

RuggedCircuits

Quote
That chip is inverting the input to the exact opposite right?


It can double and invert. You can take +5V in and put about +10V out (probably 9.5V or so). Then take that +10V and with another TC7660 (or similar) invert to -10V.

Of course, as you start drawing current from those supplies the voltages will start to creep towards 0V.

--
Check out our new shield: http://www.ruggedcircuits.com/html/gadget_shield.html

deSilva

#6
Aug 22, 2010, 04:12 pm Last Edit: Aug 22, 2010, 04:12 pm by mpeuser Reason: 1
The max232 should be 40 cents or so, albeit needing 4 10 cent caps.

You set the to drivers to +5V and ground so they provide -15V and +15V.

Please do me a favor and through all your 741 away, they will do more harm than good :-) They comsume power in addition to the outpu. That is what Low Power chips have been made for: To comsume very little on their own...

liudr

Thanks guys. How bad are the 741s? I got them new. Hate to throw new things away.

retrolefty

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Thanks guys. How bad are the 741s? I got them new. Hate to throw new things away.


They work fine, hell we won the cold war using them. There are of course much newer versions with much better specifications. Just understand the limits of it's design specs and you can use 741's for many useful circuits. It's not like they are likely to cause you to burn your house down using them. When you run out of them do research for newer designs. I like the TI TL082 FET input but there even newer better ones available. By the way here is one of my favorite op amp reference guide, also by TI: http://focus.ti.com/lit/an/slod006b/slod006b.pdf

Lefty


deSilva

#9
Aug 23, 2010, 08:53 am Last Edit: Aug 23, 2010, 08:53 am by mpeuser Reason: 1
Retrolefty - so kind of you to correct my tonge-in-cheek remarks. Please do continue! Sometimes I show off too much of my black humor...

liudr

Thanks guys. As a new grad student, I was influenced by some senior grad students' amazement with 741s when I read their parts box "They even do your laundry!". I'll buy new ones when I run out my current stock of 10 :(

Senso

The 741's are well the first commercial op amp of the history, so they are pretty bad.

retrolefty

#12
Aug 25, 2010, 12:10 am Last Edit: Aug 25, 2010, 12:11 am by retrolefty Reason: 1
Not the first, but of the first generation:

http://en.wikipedia.org/wiki/Operational_amplifier

And it mentions that the 741 is still in production, unlike many other types produced after it's arrival. So take that into consideration of proclaiming it dead and buried.  ;)

Just like hating Microsoft, there seems to be a population dedicated to hating on the poor old 741.

Lefty

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