nice amplifier i will use this in 4metal design

[simeon webber]

seasons greatings too all





http://www.mrflip.com/papers/LIA/LIAProgram/Circuit/LowNoiseAmplifier.pdf here is a usage example



it has some nice features like auto built in overvolts protections

nice gain factor x100 for micro volts inputs

and can be use in a differantial circuit or non dif circuit
one idea is to place a 4066 gate accross the differantial inputs pair {normaly a resistor in a dif circuit between pins 1 and 8


so ..... i think switched single ended /differential is best for very great depth



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[Nicko]

Re: nice amplifier i will use this in 4metal design

Hi,

Many manufacturers make instrumentation amps as a package with a single external resistor to set the gain. The main point of this is that to get very high CMRR, you need to match the resistors very accurately, typically 0.01%. In dedicated amps like this, it is done by laser trimming during manufacturing.

The real question, is why use an expensive instrmentation amp which does not have particularly good noise figures (there are many better), when the essential problem is unlikely to be common mode signals? The only common mode part of the circuit is the cable to the coil, and that can simply be screened. RFI/EMI is a much larger problem, as is thermal noise, and an instrumentation amp won;t help you there.

I looked at using a full instrumentation amp for my current design (actually one by Analog Devices), but Eric convinced me its not a path worth chasing for a variety of reasons. Instead, I'm using a very low noise opamp and paying very careful attention to component choice, auto-offset and thermal stabilisation and layout instead. The AD797AR has a typical noise figure of 0.9nVrootHz, vs. the 11 to 15uVrootHz of the amp you quote.

Nicko

[simeon.webber]

Re: nice amplifier i will use this in 4metal design

a noise figure of 11 to 15 uv this is irrelevent as the noise isnt a problem at the part of the circuit i am using it

i think you miss the point

this isnt the main opamp

i will use lm 318 or ne3342 or 5532 dual for this


it is used as part of a bigger design being switchable for diferancial
mode

nothing more.....

[Nicko]

Re: nice amplifier i will use this in 4metal design

The fact that you mentioned that

"it has some nice features like auto built
in overvolts protections" and
"nice gain factor x100 for micro volts
inputs"

implied (to me at least) that you were intending it to be used as a front-end, unless your front-end was doing little if any amplification and was outputting more than +/- 40V... ;-)

As a switchable gain amp, in common with all amps of this type, you can switch resistors in and out to change the gain - this is a standard technique. There are also rather better programmable gain amps which can be controlled by SPI or other simple means (as I presume you are using PICs) - they have an internal digital pot with up to 256 steps... these can also be single-ended or differential...

As a second stage amp, e.g. if you were intending to use a programmable gain differential amp to look at the output of two integrators, again, you can get better results with a cheaper (INA114s between $6.80 & $10 in 1-off qty) & simpler design.

Where this sort of amp wins out is in its great long-term stability, highly predictable gain and good temperature/gain behaviour.

BTW. You sent me some large files that were blocked by my firewall - what were they?

Nicko