unused quad opamp stages

[gak]

hi
i've built a morley wah with all the cmos switching and buffer stuff, but i've decided to remove it, now i want to know.. how should i connect the unused connections of the quad opamp to be used as an dual opamp...
i've connected together the inverting and the non-inverting inputs and let the output without any connection but now isn't working anymore... also, i have different voltage measures on each unused stage... i think i have done something wrong, but i can't find what it is... any hint? the way i'm connecting the opamps is the right way?

thnx!

[Peter Snowberg]

I couldn't tell you why it isn't working, but the best way to inactivate an opamp I believe is to connect the output to the inverting input, and connect the non-inverting input to ground.

Op-amps try to servo the output so that inverting input is reading the same voltage as the non-inverting input. By connecting things as above, the unused push-pull op-amp output will consume the smallest amount of current possible.

[niftydog]

i think i have done something wrong, but i can't find what it is... any hint?

go back to how it was when it WAS working and double check the op amp pin outs using the datasheet.

[frequencycentral]

....but the best way to inactivate an opamp I believe is to connect the output to the inverting input, and connect the non-inverting input to ground.

so, if the circuit is bipolar powered should I connect the non-inverting input to -15v?

[R.G.]

Yes on connecting output to (-) input. I would connect the + input to any voltage within the common mode range of the opamp.

The reason that's important is that just connecting to ground puts some opamps (the TL07x series is notable for this) outside it's input common mode range. It may be benign, but some opamps will do Ugly Things if you do this to them. Tying it to Vbias is fine. It's even OK in most cases to tie the + input to any other signal. It'll follow the signal, but it won't be out of it CMR and it's output can be safely ignored.

[frequencycentral]

Thanks RG. It's a TL084. Power supply is +/-15v. The unused opamp's non-inverting input is right next to the TL084's pin 4, ie +15v, should I connect it directly to there? Or via a resistor?

[R.G.]

Thanks RG. It's a TL084. Power supply is +/-15v. The unused opamp's non-inverting input is right next to the TL084's pin 4, ie +15v, should I connect it directly to there? Or via a resistor?

That family doesn't really like its inputs within a couple of volts of either the positive supply or negative supply. I would connect output to inverting input, and connect the + input to ground (which is in the middle of the total +/-15V supply). My comments about ground were meant for the more common pedal applications where ground is the most negative power supply. No, don't tie it to +15.

[gritz]

To confirm what RG says and to clarify for everyone:

http://www.google.co.uk/url?sa=t&rct=j&q=sloa067.pdf&source=web&cd=1&ved=0CCIQFjAA&url=http%3A%2F%2Fwww0.fh-trier.de%2F~berres%2FDatenbl%25E4tter%2FTEXAS%2Fsloa067.pdf&ei=X1EmT8XzNomO8gOHmLjLBw&usg=AFQjCNGxUIgdy6H-Yv5k9cMiUFP8eBoKiA&cad=rja

See section 4.2.1 - "Common Mistake Number 1 - Improper Termination of Unused Sections"

*crosses fingers in the hope that the link works*

[liquids]

Great reading there.

I do love the final note....I learned that really, op amps ARE awful comparators.  But comparators are useful!  So I've bought quite a few dedicated comparators....and found, for example,  that when trying to square up a signal for tracking, a true comparator makes a world of improvement in sound over an op amp rigged as a comparator, IMO.

But after spending looking at so many schematics where op amps are used as comparators, I started thinking to myself that maybe this was just unnecessarily hard line....

And then this final note:
"Comparators and op amps, although similar, are very different components. It is unlikely that a comparator will be used as an op amp. However, Texas Instruments applications have received a number of inquiries about using op amps as comparators in an open loop. The best advice Texas Instruments can give is to not do this. The very best a designer can expect is very poor performance; the worst is a circuit that does not work or even burns out."

[liquids]

Anyone care to elaborate on best practice for an unused portion of an OTA (namely 13x00 type) - would it be the same, or done differently than an unused op amp stage?

[merlinb]

Anyone care to elaborate on best practice for an unused portion of an OTA (namely 13x00 type) - would it be the same, or done differently than an unused op amp stage?

Just leave all pins unconnected. If there is no control current, the entire amp is basically off. You could connect both inputs together if it makes you feel better, but it really doesn't matter.

[R.G.]

Just leave all pins unconnected. If there is no control current, the entire amp is basically off. You could connect both inputs together if it makes you feel better, but it really doesn't matter.

Works for the OTA part of the chip and the LM13600. The LM13700 has a bias current source connected to the output darlington buffer. Much better to ground the base of that one to keep it from floating around. I'm not certain about the CA3280 and NE5517, which have similar functions. They're also much rarer.

[liquids]

The LM13700 has a bias current source connected to the output darlington buffer. Much better to ground the base of that one to keep it from floating around.

To clarify further for the 13x00 series of OTAs - it is best to ground the base of an unused 13700 darlington in all cases, or, is it best to connect it to the 'center rail' of the given power supply?   

And of note, this is one case where the 13600 and 13700 differ - if one has an unused darlington in a 13600 OTA, leave the base unconnected, but with a 13700 see the above?

I really appreciate this info.

[merlinb]

Works for the OTA part of the chip and the LM13600. The LM13700 has a bias current source connected to the output darlington buffer.

Other way around. The 13700 has an independent Darlington so you can forget about it.

In the 13600 you could ground the base of the Darlington, but it cannot come to harm even if you forget about it, provided there is no control current.

[R.G.]

I'll have to check the datasheets. My memory is that the 600 has a bias source tied to the Iabc, and the 700 has an independent bias source. In any case, grounding the base on an unused LM13x00 OTA is a simple and cheap way to make sure the darlington causes no issues in either case. Copper's cheap once you have paid for a PCB.