[sdiy] Unused opamps in multi-opamp ic / Re: triangle core VCO comparators

[Magnus Danielson]

Hi Rick,

Notice that these assumed single sided supplies, so ground here is 
really the negative rail, and running the output into the rail is NO!

The Brain-dead and Never failures relate to a story from work, where an 
ECL comparator was used for supervising a singal, but we had huge 
problems as we did not have a signal supplied to the board. Turns out 
that they had wired it up as if it was a "comparator" but they ended up 
putting it in it's linear amplification mode, so we had a wide-bandwidth 
amplifier picking up the stray signals from the board... they just 
didn't believe the analysis, as it was a "digital component". However, 
it only worked if wired up avoiding the linear amplification.

When you have a high-input impedance op-amp with a raw gain of about a 
million or so, don't let it run open-loop, and don't let it hit the rails.

Anyway, I prefer the "Good"/"Smart" method, which means for balanced 
supply that it is wired to signal ground, with some minimum setup 
allowing for a feedback resistor and such. Otherwise I try to figure out 
what the spare op-amp can do for me.

Cheers,
Magnus

On 02/25/2016 09:27 PM, Rick Jansen wrote:
> 4.2.1 in that article has an interesting section on what to do with unused opamps in a multi-opamp ic...  Comments? (Apparently I did it wrong, a few times..)
>
> rick
>
>> On 25 Feb 2016, at 21:15, Rick Jansen <rick.jansen at xs4all.nl> wrote:
>>
>> This month's (March/April) NL Elektor has an article on the difference between actual comparators and opamps. True comparators are (much) faster, and can reach rail voltages, and so not rely on the amplification factor of the opamp. Also, the output stage of many opamps is not suited for comparator functions, and can even damage them.
>>
>> The end of the article refers this document: http://encon.fke.utm.my/nikd/latest/sloa067.pdf
>>
>> Bruce Carter : opamps and comparators , don't confuse them.
>>
>> R.
>>
>>> On 18 Feb 2016, at 01:48, Chris McDowell <declareupdate at gmail.com> wrote:
>>>
>>> howdy,
>>>
>>> I'm wondering about the choice of comparator for triangle core VCOs. I've seen it done a few ways, like Thomas Henry's discrete comparator, or the LM311 in J3RK's (STROH Modular) VCOs on Muffs, and my own builds just use TL07x and work -pretty much- great. But I'm wondering what the experiences and preferences are of you here on the list. Fast op amp? Real comparator? Doesn't matter? I'm aware that it only matters as much as HF-Tracking matters, but I'm interested in opinions anyway.
>>>
>>> Cheers!
>>> Chris
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