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Elementary questions about electronics
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Special op amp "tricks"
Are there any tricks I need to be aware of for practical op amp circuits? Mainly stuff to do with audio? Just wondering how close the textbook stuff is to reality?
Thanks,
Jon
On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter" <Jon_Slaugh @Hotmail.com> wrote: >Are there any tricks I need to be aware of for practical op amp circuits? >Mainly stuff to do with audio? --- Yes. 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
resistors connected to the opamp inputs as close to the opamp as
you can get them. Sacrifice almost everything for that.
Ditto for what's driving the resistors through which the opamp is
being driven, but the opamp input end of the resistors takes
precedence.
2. Always roll off any bandwidth you don't need.
>Just wondering how close the textbook stuff is to reality?
--- Unless otherwise indicated, the textbook stuff will mostly be sunshine on a cloudy day. --
JF
-----------------------------------------------Reply-----------------------------------------------
"John Fields" <jfie @austininstruments.com> wrote in message ... > On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter" > <Jon_Slaugh @Hotmail.com> wrote: >>Are there any tricks I need to be aware of for practical op amp circuits?
>>Mainly stuff to do with audio?
> ---
> Yes.
> 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
> resistors connected to the opamp inputs as close to the opamp as
> you can get them. Sacrifice almost everything for that.
Is this mainly for HF stuff or does it matter. What is the purpose of this too? lead inductance? > Ditto for what's driving the resistors through which the opamp is
> being driven, but the opamp input end of the resistors takes
> precedence.
huh? Not sure what you mean here? How can you do this if the signal is, say, comming from a guitar or mic? > 2. Always roll off any bandwidth you don't need.
And this is because of oscillations? Thanks,
Jon
-----------------------------------------------Reply-----------------------------------------------
On Fri, 11 May 2007 22:25:32 GMT, "Jon Slaughter"
<Jon_Slaugh @Hotmail.com> wrote: >"John Fields" <jfie@austininstruments.com> wrote in message
>news:ph5743dbogikb0dlpta2vpifggescm7hg1@4ax.com...
>> On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter"
>> <Jon_Slaugh@Hotmail.com> wrote:
>>>Are there any tricks I need to be aware of for practical op amp circuits?
>>>Mainly stuff to do with audio?
>> ---
>> Yes.
>> 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
>> resistors connected to the opamp inputs as close to the opamp as
>> you can get them. Sacrifice almost everything for that.
>Is this mainly for HF stuff or does it matter.
--- It's for everything. --- >What is the purpose of this too? lead inductance?
--- long leads are antennas, so you wind up picking up all manner of crap including, possibly, signal from the output and Voila! oscillator. --- >> Ditto for what's driving the resistors through which the opamp is
>> being driven, but the opamp input end of the resistors takes
>> precedence.
>huh? Not sure what you mean here? How can you do this if the signal is, say,
>comming from a guitar or mic?
--- Make the leads (or traces) from the input connectors of your widget to the circuitry as short as possible. --- >> 2. Always roll off any bandwidth you don't need. >And this is because of oscillations?
--- And noise. --
JF
-----------------------------------------------Reply-----------------------------------------------
On May 10, 6:27 pm, John Fields <jfie @austininstruments.com> wrote:
> On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter" > <Jon_Slaugh@Hotmail.com> wrote:
> >Are there any tricks I need to be aware of for practical op amp circuits?
> >Mainly stuff to do with audio?
> ---
> Yes.
> 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
> resistors connected to the opamp inputs as close to the opamp as
> you can get them. Sacrifice almost everything for that.
> Ditto for what's driving the resistors through which the opamp is
> being driven, but the opamp input end of the resistors takes
> precedence.
> 2. Always roll off any bandwidth you don't need.
> >Just wondering how close the textbook stuff is to reality?
> ---
> Unless otherwise indicated, the textbook stuff will mostly be
> sunshine on a cloudy day.
> --
> JF
Amen to #1. Possibly that could be expanded to include the feedback loops -- keeping them as small as possible and putting the terminating leads as close as possible to the input pins also works miraculous wonders. In addition to #2, a couple more lesser caveats might be in order:
* Always bypass the power supply for anything faster than an LM358
(and sometimes even then), and keep the AC impedance of the bypasses
to GND as small as possible. Memorize the venerable Analog Devices
appnote AN-202, "An IC Amplifier User's Guide to Decoupling,
Grounding, and Making Things Go Right for a Change". This appnote was
written in the days when dinosaurs ruled the earth and AD still made
modular op amps, but its advice is still just as valid.
http://www.analog.com/UploadedFiles/Application_Notes/135208865AN-202...
* Always look carefully at fault conditions, and make absolutely sure
the input voltages will never exceed absmax under any circumstances.
Again, careful planning with GND impedances is necessary. An op amp
input can be grievously damaged, but still kinda function, causing no
end of headaches.
Textbook stuff is great as far as it goes. But the OP might just want
to open up good audio equipment, especially older stuff from the
seventies and eighties, and look carefully at their schematics. The
additional practical methods to make things work well and reliably are
right in front of him, if he takes the time and effort to learn from
them.
Cheers
Chris
-----------------------------------------------Reply-----------------------------------------------
"John Fields" <jfie @austininstruments.com> wrote in message ...
> On Fri, 11 May 2007 22:25:32 GMT, "Jon Slaughter" > <Jon_Slaugh @Hotmail.com> wrote: >>"John Fields" <jfie@austininstruments.com> wrote in message
>>news:ph5743dbogikb0dlpta2vpifggescm7hg1@4ax.com...
>>> On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter"
>>> <Jon_Slaugh@Hotmail.com> wrote:
>>>>Are there any tricks I need to be aware of for practical op amp
>>>>circuits?
>>>>Mainly stuff to do with audio?
>>> ---
>>> Yes.
>>> 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
>>> resistors connected to the opamp inputs as close to the opamp as
>>> you can get them. Sacrifice almost everything for that.
>>Is this mainly for HF stuff or does it matter.
> ---
> It's for everything.
> ---
>>What is the purpose of this too? lead inductance?
> ---
> long leads are antennas, so you wind up picking up all manner of
> crap including, possibly, signal from the output and Voila!
> oscillator.
> ---
>>> Ditto for what's driving the resistors through which the opamp is
>>> being driven, but the opamp input end of the resistors takes
>>> precedence.
>>huh? Not sure what you mean here? How can you do this if the signal is,
>>say,
>>comming from a guitar or mic?
> ---
> Make the leads (or traces) from the input connectors of your widget
> to the circuitry as short as possible.
> ---
>>> 2. Always roll off any bandwidth you don't need.
>>And this is because of oscillations?
> ---
> And noise.
Thanks again, I'll try to keep that stuff in mind next time I play with them but chances are I'll forget ;/ Jon
-----------------------------------------------Reply-----------------------------------------------
"Chris" <cfoley1 @yahoo.com> wrote in message ...
> On May 10, 6:27 pm, John Fields <jfie @austininstruments.com> wrote: >> On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter" >> <Jon_Slaugh@Hotmail.com> wrote:
>> >Are there any tricks I need to be aware of for practical op amp
>> >circuits?
>> >Mainly stuff to do with audio?
>> ---
>> Yes.
>> 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
>> resistors connected to the opamp inputs as close to the opamp as
>> you can get them. Sacrifice almost everything for that.
>> Ditto for what's driving the resistors through which the opamp is
>> being driven, but the opamp input end of the resistors takes
>> precedence.
>> 2. Always roll off any bandwidth you don't need.
>> >Just wondering how close the textbook stuff is to reality?
>> ---
>> Unless otherwise indicated, the textbook stuff will mostly be
>> sunshine on a cloudy day.
>> --
>> JF
> Amen to #1. Possibly that could be expanded to include the feedback
> loops -- keeping them as small as possible and putting the terminating
> leads as close as possible to the input pins also works miraculous
> wonders.
> In addition to #2, a couple more lesser caveats might be in order:
> * Always bypass the power supply for anything faster than an LM358
> (and sometimes even then), and keep the AC impedance of the bypasses
> to GND as small as possible. Memorize the venerable Analog Devices
> appnote AN-202, "An IC Amplifier User's Guide to Decoupling,
> Grounding, and Making Things Go Right for a Change". This appnote was
> written in the days when dinosaurs ruled the earth and AD still made
> modular op amps, but its advice is still just as valid.
> http://www.analog.com/UploadedFiles/Application_Notes/135208865AN-202...
> * Always look carefully at fault conditions, and make absolutely sure
> the input voltages will never exceed absmax under any circumstances.
> Again, careful planning with GND impedances is necessary. An op amp
> input can be grievously damaged, but still kinda function, causing no
> end of headaches.
> Textbook stuff is great as far as it goes. But the OP might just want
> to open up good audio equipment, especially older stuff from the
> seventies and eighties, and look carefully at their schematics. The
> additional practical methods to make things work well and reliably are
> right in front of him, if he takes the time and effort to learn from
> them.
yeah, I never thought to do that. But then again, its one thing to see what they were doing and totally different to understand why they did it. Thanks,
Jon
-----------------------------------------------Reply-----------------------------------------------
On May 11, 7:20 am, "Jon Slaughter" <Jon_Slaugh @Hotmail.com> wrote: > Are there any tricks I need to be aware of for practical op amp circuits?
> Mainly stuff to do with audio?
I've seen op amps used in parallel to reduce noise. Neat trick. Dave.
-----------------------------------------------Reply-----------------------------------------------
On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter" <Jon_Slaugh @Hotmail.com> wrote: >Are there any tricks I need to be aware of for practical op amp circuits? >Mainly stuff to do with audio? >Just wondering how close the textbook stuff is to reality?
>Thanks,
>Jon
One trick that you are likely to need for audio use is the ability to drive a cable. Many op-amps are poor at this due to the capacitance of the cable. The trick is to put 100 ohms between the amp output and the cable, and connect the amp feedback resistance from the cable side of the resistor instead of from the amp output. You also need a small feedback cap (10-50 pF) wired in the usual way from output to - input. Best regards.
Bob Masta
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!
-----------------------------------------------Reply-----------------------------------------------
On Sat, 12 May 2007 18:43:01 GMT, "Jon Slaughter"
<Jon_Slaugh @Hotmail.com> wrote: >"John Fields" <jfie@austininstruments.com> wrote in message
>news:9mt94392ts1noaei341nmgtjpn6ben1fsa@4ax.com...
>> On Fri, 11 May 2007 22:25:32 GMT, "Jon Slaughter"
>> <Jon_Slaugh@Hotmail.com> wrote:
>>>"John Fields" <jfie@austininstruments.com> wrote in message
>>>news:ph5743dbogikb0dlpta2vpifggescm7hg1@4ax.com...
>>>> On Thu, 10 May 2007 16:20:05 -0500, "Jon Slaughter"
>>>> <Jon_Slaugh@Hotmail.com> wrote:
>>>>>Are there any tricks I need to be aware of for practical op amp
>>>>>circuits?
>>>>>Mainly stuff to do with audio?
>>>> ---
>>>> Yes.
>>>> 1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the
>>>> resistors connected to the opamp inputs as close to the opamp as
>>>> you can get them. Sacrifice almost everything for that.
>>>Is this mainly for HF stuff or does it matter.
>> ---
>> It's for everything.
>> ---
>>>What is the purpose of this too? lead inductance?
>> ---
>> long leads are antennas, so you wind up picking up all manner of
>> crap including, possibly, signal from the output and Voila!
>> oscillator.
>> ---
>>>> Ditto for what's driving the resistors through which the opamp is
>>>> being driven, but the opamp input end of the resistors takes
>>>> precedence.
>>>huh? Not sure what you mean here? How can you do this if the signal is,
>>>say,
>>>comming from a guitar or mic?
>> ---
>> Make the leads (or traces) from the input connectors of your widget
>> to the circuitry as short as possible.
>> ---
>>>> 2. Always roll off any bandwidth you don't need.
>>>And this is because of oscillations?
>> ---
>> And noise.
>Thanks again, I'll try to keep that stuff in mind next time I play with them
>but chances are I'll forget ;/
--- Oh, don't worry, get bit on the ass enough times and it'll become part of your most easily accessible _permanent_ memory! ;) --
JF
-----------------------------------------------Reply-----------------------------------------------
"Jon Slaughter" <Jon_Slaugh @Hotmail.com> wrote in message ... > Are there any tricks I need to be aware of for practical op amp circuits? > Mainly stuff to do with audio? > Just wondering how close the textbook stuff is to reality?
> Thanks,
> Jon
Thanks guys. I'll try to keep all this stuff in mind but atleast I know some of the details. Jon
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