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Electronics => Beginners => Topic started by: ramonest on July 29, 2017, 07:07:10 pm

Title: Dual OP Amp implementation Doubt
Post by: ramonest on July 29, 2017, 07:07:10 pm
Hello, I was studying some schematics. There was a dual OP amp IC being used. One of the internal amplifiers was being used as (I belive) signal conditioning before doing a measure with an ADC. I think I understand that part.
What I don't understand is that the other OP amp inside the IC was connected: non-inverting to GND, and a 100 kOhm between inverting and output. It isn't connected to anything else, ideally it has no function, but I guess that if they did that it's for a reason. Anyone knows why?
(I probably didn't explain myself too well so I'll add an schematic with the circuit.) Dual OP Amp IC is a TL082 if that's usefull.

Thanks!
Title: Re: Dual OP Amp implementation Doubt
Post by: Neilm on July 29, 2017, 07:15:47 pm
It has been connected like that to stop it floating. You don't want chips to be unconnected as they can end up oscillating and causeing all sorts of problems.
Title: Re: Dual OP Amp implementation Doubt
Post by: John B on July 29, 2017, 11:04:10 pm
http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx (http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx)

This covers some of the issues.
Title: Re: Dual OP Amp implementation Doubt
Post by: Lee Leduc on July 29, 2017, 11:16:10 pm
TI has a good video about this.
https://www.youtube.com/watch?v=gwQiFuckMz0 (https://www.youtube.com/watch?v=gwQiFuckMz0)
Title: Re: Dual OP Amp implementation Doubt
Post by: ramonest on July 30, 2017, 12:01:25 pm
That was helpful thank you all!
Title: Re: Dual OP Amp implementation Doubt
Post by: David Hess on July 30, 2017, 11:55:15 pm
There are a couple of other reasons this is important:

1. Terminating the amplifier within its input and output voltage ranges prevents unusual operation and saturation of any stage.
2. Some dual and quad parts share bias stages and operating one beyond its specifications or saturating it will affect the others.

Sometimes you can break the rules and shut an amplifier off by pulling its inputs outside of its input common mode range but this depends on the design.
Title: Re: Dual OP Amp implementation Doubt
Post by: retrolefty on July 31, 2017, 12:01:10 am
Of course the same issue applies to CMOS logic chips, unused inputs should be pulled-up/down to prevent problems. Most modern microcontroller chip these days offer programmable pull-up and or down for input pins, making a no cost solution.

Title: Re: Dual OP Amp implementation Doubt
Post by: electrolust on July 31, 2017, 06:24:17 pm
Quote from: Lee Leduc on July 29, 2017, 11:16:10 pm
TI has a good video about this.
Code: [Select]
https://www.youtube.com/watch?v=gwQiFuckMz0

unfortunate URL encoding on this one
Title: Re: Dual OP Amp implementation Doubt
Post by: bson on August 01, 2017, 07:10:23 am
Quote from: electrolust on July 31, 2017, 06:24:17 pm
Quote from: Lee Leduc on July 29, 2017, 11:16:10 pm
TI has a good video about this.
Code: [Select]
https://www.youtube.com/watch?v=gwQiFuckMz0

unfortunate URL encoding on this one
:-DD