Operational Amplifiers - unused amps

[jedynakiewicz]

I am using an LM324 Operational Amplifier IC to handle the DAC output from a PICAXE. I only need one of the four operational amplifiers on the chip; I am using pins 1,2,3,4 &11. What should I do with the unused pins? If I leave them unconnected I fear that the chip may be damaged by static etc. Should the inputs be tied to ground and the outputs left open? Expert advice would be much appreciated.

 

[nick12ab]

http://forum.allaboutcircuits.com/showthread.php?t=37694

 

[jedynakiewicz]

Gosh, Nick12ab, thanks for that - its quite an involved an issue as I thought it might be. The onward link to http://pdfserv.maximintegrated.com/en/an/AN1957.pdf is Maxim's tech note on what to do. Most helpful. I shall read it in detail.

Nevertheless, if any forum experts have any further indication as to what they have or have not done I would be most interested to hear.

 

[jedynakiewicz]

Okay, I think that I am sorted. Herewith a reference diagram for anyone else looking for such an answer. I have patched in the terminations for amplifier 2 in the chip. The Maxim note does not give any resistor values - I am presuming that 10K should probably be right? Anyone got any advice/opinion on that?

 

[MartinM57]

Try again - non-inverting (+) input to junction of resistors

 

[BeanieBots]

An LM324 and you're only using 1/4 of it! There are countless 8-pin op-amps out there and even some of those are dual.
Anyway, unused inputs. There were quite a few opinions on those links and some of them were worse than leaving the inputs floating.
You should tie the -ve input to the output and the +ve input to mid potential. As the LM324 struggles to get within a few volts of its rails I assume you are using dual rails, so in your case, that would be 0v.
(or as MartinM57 describes about your diagram)
When prototyping I never bother and have never had any issues. The LM324 is a tough old beast and one of my favourites. (as it has 4 in one package).

 

[jedynakiewicz]

Try again - non-inverting (+) input to junction of resistors

I was just checking to see if you spotted my deliberate error...well, okay I cannot believe that I messed up the diagram- thanks for that (shows what happens when you sketch up whilst hurrying to get ready for Church on Sunday). My post now edited to correct the configuration.

 

[dmaxben]

For what its worth....I just use an MCP6021. Small (SOT-23 or 8-pin DIP), only has one op-amp, and works perfectly with the Picaxe DAC...

ben