rail to rail opamps

[Brian Marshall]

Anyone know of some good, but not overly expensive quad (or dual) opamps that will get pretty close to the rails.  

minimum cross talk is important.  
slew rate should not be horribly bad, but not sure it will really matter much as I'm not doing any high frequency stuff.

noise not really important.

[niftydog]

didn't delve too much into the detail, but these look ok.


Maxims website seems like it's down, but they have a pretty good range. They can be pricey though, but you can buy direct from them.

Also, they'll send you two for free!

[Brian Marshall]

they dont give you much of a discount for buying bulk either.

I was expecting rail to rail opamps to be about $1 each, but i figured they'd go down fast once you bought 100 of them.

[gez]

ICL7611 single
ICL7621 dual
ICL7631...you get the picture.

For a really classy audio amp I use CA3130s and for LFOs etc I use one of the above or the LMC660 quad.  

Apart from the CA3130 (which is exceptionally good for a CMOS amp) the slew rates of the above are pretty poor, but that's often not a problem for most of the circuits that get built around here.

All of the above have rail-to-rail output swing.

[bwanasonic]

Check out Analog Devices:

http://www.analog.com/Analog_Root/sitePage/mainSectionHome/0%2C2130%2Clevel4%253D%25252D1%2526Language%253DEnglish%2526level1%253D150%2526level2%253D151%2526level3%253D155%2C00.html

Have no idea about cost though.

Kerry M

[Hal]

TI has them...
http://focus-webapps.ti.com/general/docs/sitesearch/searchsite.tsp;jsessionid=1RPGPORIQA0BZQC1JAWB3KQ?selectedTopic=1653260327&numRecords=25&searchTerm=rail+to+rail&statusCode=null

i see OPA347 as something that might be ok....

be good to them, they're good to us :-D

[Ansil]

what about the tlc2262??? and 2264

[Brian Marshall]

One thing i think i should explain a little better.

I will be using 3 of the four as unity buffers for control voltage.  so if the input goes down to 0v the output has to go down to 0v or at least as close as it can get to it.

Using a tl074, as soon as the input voltage goes below the rail, which is a little over one volt, the output saturates to the positive rail....  

I'm starting to think that i may need to just get away from opamps completely.

[mikeb]

I'm starting to think that i may need to just get away from opamps completely.

It depeds on what it is you're trying to achieve. It's always possible to scale and amplitude-transform voltages on input and reverse these transformations on output; inbetween times you do whatever you need to do with the voltage.

CV guidelines:
- someone has done it before you
- that someone likely has some info on the net and failing that...
- that info is available in books

Maybe you need to take a step back and analyse what it is you are trying to achieve, and change how you do this based on what restrictions you have to hand (9V single-ended power supply, type of opamps etc).

Cheers

Mike

[Brian Marshall]

I'm starting to think that i may need to just get away from opamps completely.

It depeds on what it is you're trying to achieve. It's always possible to scale and amplitude-transform voltages on input and reverse these transformations on output; inbetween times you do whatever you need to do with the voltage.

CV guidelines:
- someone has done it before you
- that someone likely has some info on the net and failing that...
- that info is available in books

Maybe you need to take a step back and analyse what it is you are trying to achieve, and change how you do this based on what restrictions you have to hand (9V single-ended power supply, type of opamps etc).

Cheers

Mike

I know what you are trying to say mike.  I could certainly limit the input voltage from falling below 1.5 volts or so.  basically i have 3 different control voltages that outupt through a resistor network.  making lots of voltage dividers could fix it, but the circuit would grow quite a bit larger than i feel it really needs to be.  then there is all the variances in components that could get me in trouble.  im thinking bipolar transistors may work a lot better for these three buffers, but i fear that the reduced output impedance may be a problem.

and mike, you are right there is a lot of info on the net, but a lot of the stuff on the net takes a while to find, and sometimes things that at first seem aplicable to a given problem end up not being as useful as you might think.  I have read a few books, and a lot of data sheets, and i have never seen anything about an opamp saturating to the opposite rail when the noninverting input falls outside the opamps output voltage range.

[mikeb]

Look here:
http://www.analog.com/library/analogDialogue/Anniversary/6.html
and find the bit titled "Q. What other features of op amps should the user know about?"

This behaviour is one of the reasons why a plain-and-simple 4558 is used in some envelope filters (eg Dr Q, MutronIII) - other devices (eg TL072) won't operate correctly under the conditions set by the circuit.

Cheers

Mike

[Brian Marshall]

thanks mike.  I appreciate it.

brian

by the way, i did some experimenting with bipolars, and it looks promising. it may also make designing a board arround it a little easier.

[Brian Marshall]

and there's my answer

A. A common problem encountered with JFET op-amps is phase inversion. If the input common-mode voltage of a JFET op-amp approaches the negative supply too closely, the inverting and non-inverting input terminals reverse functions. Negative feedback becomes positive feedback and the circuit may latch up. This latchup is unlikely to be destructive, but power may have to be switched off to correct it. This figure shows the effect of such phase inversion in a circuit where latch-up does not occur. The problem may be avoided by using bipolar amplifiers, or by restricting the common-mode range of the signal in some way.

[mikeb]

No problems ..... I got lucky with the web searching on that one!

Mike

[Brian Marshall]

Hey mike, this little project of mine has been a real learning experience.  it has gone from a 3 pots to 8, and has been re-designed 3 times from the ground up.  about the only thing that is the same from the beginning is the input buffer.

I think some time soon, i am going to replace most of the opamp stages with bipolar transistors.  I was considering using an optoisolator instead of the LED, in the filter section, but i just discovered a new way to control the range of the circuit by varying the current instead of the voltage.  Diodes are soooo cool.  I guess i didnt actually learn a new fact, but a new way to apply a rule that i already knew.  It seems so obvious.  Sometimes the greatest revelations are made by just sitting down and working on something long enough to really get you thinking about what it is that you are doing.

Infact in the last two weeks I've learned ways to apply the rules in ways i wasnt aware of before.  I'm certain none of it is earth shattering, but it feels good getting a better grasp on how to accomplish certain things.