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Old 22nd May 2015, 09:30 AM   #1
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Default OpAmp Input Bias Current Return Path?

Several of the opamps I have been studying require a resistor from either or both inputs to ground to prevent the inputs from floating to a potential which exceeds the common mode range, causing the input amplifiers to saturate.

But won't that cause a voltage divider on the inputs, thus reducing accuracy?

My main goal here is to find an opamp with zero drift and as low offset as possible. I will be using it at DC so I don't need any bandwidth.

What I have discovered is that the TL071 I have been using has terrible drift and poor temperature coefficient.
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Old 22nd May 2015, 10:20 AM   #2
DF96 is offline DF96  England
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An opamp needs a DC path for each input. It is a matter of good circuit design to ensure that this is provided in a way which does not disrupt the required circuit operation. Many opamps need similar DC resistance at both inputs. TL071 may not, as I believe it is JFET input.
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Old 22nd May 2015, 10:32 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

Don't know much about Op-Amps for DC, chopper stabilised and the like.
I do know audio opamps are generally useless for precision DC work.

rgds, sreten.
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Old 22nd May 2015, 10:37 AM   #4
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Quote:
Originally Posted by sreten View Post
Hi,

Don't know much about Op-Amps for DC, chopper stabilised and the like.
I do know audio opamps are generally useless for precision DC work.

rgds, sreten.
Useless?
Speak to the industry standard designers at Crown and for that matter most all designers using DC coupled amplifiers.
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Old 22nd May 2015, 11:03 AM   #5
Did it Himself
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DC coupled audio power amps are different beasts to precision DC amplifiers...
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Old 22nd May 2015, 11:27 AM   #6
Mooly is offline Mooly  United Kingdom
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It would help to have some idea of the circuit details. To describe the drift of the TL071 as 'terrible' needs putting in context. Are we talking microvolt resolution here... what ?
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Old 22nd May 2015, 11:36 AM   #7
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Quote:
Originally Posted by johngalt47 View Post
Several of the opamps I have been studying require a resistor from either or both inputs to ground to prevent the inputs from floating to a potential which exceeds the common mode range, causing the input amplifiers to saturate.

But won't that cause a voltage divider on the inputs, thus reducing accuracy?

My main goal here is to find an opamp with zero drift and as low offset as possible. I will be using it at DC so I don't need any bandwidth.

What I have discovered is that the TL071 I have been using has terrible drift and poor temperature coefficient.
As usual in engineering, it depends! And you need to do some numbers in the light of your application.

What are the impedances involved? Zero drift does not exist, so how low can you get away with?

This is not an audio problem, but an instrumentation one. If impedances are lowish, you can use a chopper stabilised amp, but they have their own problems.

If you baseline is TLo7x series, a cheapo cooking OA, you ae in the wrong league. Try looking at OP07 and later derivatives.

As always, detail your application for less guesswork and more help!
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