Let's Make Robots!

Op Amps - why/how?

Hi All,

I've been sniffing around the Picaxe forum for some tips/ideas, and I noticed that almost every project they have uses some sort of Op Amp.

Can anyone give me any pointers on using an Op Amp with a Picaxe?

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If you're really into analogue electronics, op-amps and the like, there's an excellent textbook by Walt Jung: IC Op-Amp Cookbook.  It's quite an old book now, but there are several newer editions and I think it's still in print.  If you know of a good source for secondhand books, look out for the others in the series: Audio IC Op-Amp Applications, IC Timer Cookbook (all about the 555 and similar chips), IC Converter Cookbook (all about DAC and ADC circuits), and a couple of others.

 

Would that be the Op Amp Applications Handook? It's mighty expensive even second hand on Amazon! I wonder if it would be cheaper in a book shop near the Uni.

But, I've just discovered that (allegedly) you can download it for free from the Analogue Devices website!

Mike

I think that downloadable book is a sort of composite of two books in the original series.  Looks like Analog Devices has bought the rights to offer it in that way, so well worth downloading a copy.  The Nat Semi and TI PDF files are good, too.  If you want to read about the history of the Op-Amp (back into the 1950s), look up the archives of Philbrick Researches:

 http://www.philbrickarchive.org/

TI has a very nice document on the theory and application of op amps if you're interested: http://focus.ti.com/lit/an/sboa092a/sboa092a.pdf

 If you're using a robot, the dual supply might be a problem.  But if I need a single supply op amp, I usually use the LM358

Gabe

Oddbot is a great example of why this website is AWESOME.

Thanks a ton. 

Opamps are a versatile device, they can not only be used as amplifiers but also for voltage addition, subtraction, comparison. They can even be used as an oscillator or integrator. The output would normally go directly to a processors analog input.

The biggest limitation is that most opamps like to have a +/-V supply totalling as much as 36V although their minimum supply requirements are usually about half of that. Of the few that can run on five volts it can be difficult to get their output to swing all the way from 0 to 5V. The CA3130 will run at 5V with 0-5V output but is prone to noise in some applications. I used one in the LaserRF MkII to boost the output from my hall effect sensor to increase resolution.

Page 12 of this data section from a Dick Smith catalog list a good selection of opamps. It list many common types along with their voltage requirements, pin connections, bandwidth etc. makes it easy to see at a glance which opamp might suit your needs.

If you do need an opamp with higher voltage requirements then my recent post on DC-DC converters shows a relatively cheap IC that can be used for stepping 5V up or even inverting it for a -5V supply. Another option is a max232 or similar RS232 interface IC as it will generate a +10/-10 supply from a single 5V source that is powerful enough to run some opamps and you get an RS232 interface as a bonus. Just make sure to protect your analog input from voltages outside of its range. A 1K series resistor and a 5.1 volt zener diode would be a good choice.