Concerns with OPA4227 Low input voltage - diyAudio
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Old 17th July 2004, 12:49 AM   #1
Beggar is offline Beggar  United Kingdom
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Default Concerns with OPA4227 Low input voltage

Hi

I've managed to get hold of some OPA4227's, intended for use in an active crossover, however whilst looking over the datasheet i was a little troubled when i noticed that the maximum signal input is +-0.7v!
I'm reasonably sure line level is around 1v, maybe a bit more sometimes.

Could anyone tell me, if i need to put a signal attenuator before these things, and increase the gain of my amps, or if they will be 'fine' without anything?

Also i have to add a 500ohm resistor between the output and inverting input when used in unity gain as in the crossover (linkwitz 24db as here), logic tells me that this wont affect the filter freqency/phase response at all, but then my knowlegde and logic are sketchy at best, could anyone confirm this for me?

One last thing, these things are sounding like they might be more effort than they are worth, i really dont want to go back to TL074's so if anyone could suggest a replacement quad op amp which wont cost me the earth (I'll need 5!) and is availble in SO14 i would be very grateful

Thanks, Nik
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Old 17th July 2004, 05:01 AM   #2
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Default Re: Concerns with OPA4227 Low input voltage

Quote:
Originally posted by Beggar
the maximum signal input is +-0.7v!
Look at the data sheet again.
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Old 17th July 2004, 09:41 AM   #3
Beggar is offline Beggar  United Kingdom
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Default Re: Re: Concerns with OPA4227 Low input voltage

Quote:
Originally posted by Bill Fitzpatrick


Look at the data sheet again.
Hello, well im not desperately sure what im looking for, but i did certainly look again. And under absolute maximum ratings it says:
Quote:
Signal Input Terminals, Voltage ........................ (V) 0.7V to (V+) +0.7V
So this is less than line level? Could you maybe be a little more specfic with what im looking for in the datasheet?

Thanks, Nik.
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Old 17th July 2004, 10:28 AM   #4
Beggar is offline Beggar  United Kingdom
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ok so ive seen the same thing 0.7v written on other datasheets, and ive never heard about this causing any issues, this is strongly suggesting stupidity / ignorance on my part causing the problems.

Maybe the question should be more directed as "I have no clue why this says 0.7v here, please help," also i know in the datasheet input voltage under common mode operation it quotes +-2v, so now i am totally confused?
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Old 17th July 2004, 12:36 PM   #5
Beggar is offline Beggar  United Kingdom
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Unhappy Oh dear i feel quite stupid

Hi i've worked out what the (v-) -0.7v etc means so it now conforms to how i think opamps should/do work. Only now do i see what Bill meant earlier

I would however, still like to know if people agree with me that the a current limiting resistor of 500ohm placed in the negative feedback path would not alter the crossover frequency or phase response of the filter stages in the crossover linked to earlier.

Sorry about all this, Nik
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Old 17th July 2004, 02:42 PM   #6
jcx is online now jcx  United States
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the spec you cite is a limitation of the input voltage at either input to a diode drop above or below the supply rails, not to worry assuming you are actually using a power supply- unless you have a low Z input that may be operating when the op amp power supply is off that can push more than 20 mA into the chip

the 500 Ohm unity feedback R is unesscessary in audio, line level AUDIO source will not exceed the 10 V/uS slew limit of the opa227, further, the abs max 20 mA spec still applies, no damage can occur if the source cannot supply more than 20 mA through any filter input R
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Old 17th July 2004, 09:35 PM   #7
Beggar is offline Beggar  United Kingdom
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Hi jcx

Thanks very much for clearing all this up for me
Time to get soldering now!

Nik
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