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In-amp noise specs

I was thinking of using an instrumentation amp instead of a normal op amp for a x10 gain stage, which has to be low voltage noise, because I wanted both inputs to be high impedance (I also considered a simple voltage follower on one input). I noticed something about the noise specifications I did not understand and hoped someone here could advise what is going on.

The normal op amp I originally considered is an LT6202 from Linear Tech, which I've found to be a very stable, easy to use device in the past. It has a 100MHz bandwidth and an input noise spec of 2.8nV/rt Hz. This is presumably 28nV/rt Hz and a bandwidth of 10MHz at an (inverting) gain of

10 by the time it gets to the output.

Comparing this with Analog Devices' AD8253 in-amp, I see its noise is specified as 45nV/rt Hz at a gain of 1 (alas at a lower frequency so one cannot truly directly compare!) but only 12nV/rt Hz RTI at a gain of 10. Does this mean that this in-amp's noise is LOWER at higher gain (presumably due to some feedback trick) and thus better than the simple op amp's? Or does it mean the voltage noise at a gain of 10 is 120nV / rt Hz at the output and hence much worse than the normal op amp's?

It also strikes me that the normal op amp's noise specs probably don't include the thermal noise from feedback resistors, so the 2.8nV/rt Hz figure is optimistic, whereas the in-amp's noise figure will be a truer reflection of what's achievable.

Since noise types vary with frequency, it is relevant to note the circuit needs to pass signals from DC up to 300kHz, maybe a little higher. So I'm concerned about low frequency (DC) drift as well as the overall AC fuzz emerging from the stage. Input levels will be about 20mV max.

Thank you,

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Nemo
Reply to
Nemo
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Nemo a écrit :

First, instrument amplifiers difference stage are noisy. When you put some gain in the front end, the ***RTI*** noise contribution from the difference output stage becomes lower WRT to the input stage contribution.

Second, to compare circuits noise you need to choose one reference point (either input or output). It's customary to refer noise to the input because this allow easy comparison, independently of the following stages' gain.

Your LT6202 at 2.8nV/rtHz RTI noise has, as you say, 28nV/rtHz output noise.

The AD8253 with 12nV/rtHz RTI noise has 120nV/rtHz output noise.

And yes, you have to add the external circuitry own noise. Just remember that you sum noise quadratically, and also don't forget the opamp input current noise contribution into the feedback and source impedances. This might be higher than your opamp voltage noise.

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Thanks,
Fred.
Reply to
Fred Bartoli

That's very clear. Thank you, Fred.

--
Nemo
Reply to
Nemo

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"and also don't forget the opamp input current noise contribution into the feedback and source impedances. This might be higher than your opamp voltage noise."

Absolutely! Since the OP mentioned high input impedances. What's your input impedance and the current noise of the above opamps.

George H.

Reply to
George Herold

At low gains you also have to worry about the inamp's output stage noise, which is usually specified separately (and is also usually horrible).

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal
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Reply to
Phil Hobbs

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Ahh I'd never thought of that. I'm almost always using Inamps at high gain. Bridge circuits and such. You can always 'roll your own' inamp with a quad opamp.

George H.

Reply to
George Herold

"Nemo"

** A 630 ohm resistor has a noise voltage of 2.8nV/rtHz.

You say your source is high impedance - so you whole post is bollocks.

.... Phil

Reply to
Phil Allison

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