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Why do PGAs have so much noise ?

Question asked by tk17171 on Feb 2, 2018
Latest reply on Feb 19, 2018 by Emman.A

Hi, I am an not a professional el. eng., so I might have missed something.

 

I am developing an application where I need selectable preamplification before running into a differential ADC input.

 

The Signal is symmetrical, but can contain a Little bit of common mode voltage. Everything is within +/- 10 V, so a +/- 15 V supply will be used for the analog chain. Frequency rate is around dc-20khz. The Signal impedance can be diverse so a high impedance Input is needed and I cannot directly use a fully differential amplifier.

 

Many programmable gain amps seem to be in-amps, but it would be actually better to maintain a differential Output after preamplification, so simple dual Non-inverting Opamps should be perfect, bringing me to my first question:

 

1) Why are there so few Standard PGAs ? There seem to be many more PGIAs nowadays. Are (dual) PGAs bad for some reason? To me, they look like the obvious choice when you want high Input impedance and differential Output.

 

After finding only PGAs with 10nV/rtHz and more noise (best was AD8253 iirc), I started wondering where are the good amplifiers. Because Standard Inamps are available with 10x less noise. so here goes q. no. 2.

 

2) Why aren't there any programmable gain amps (doesnt matter if InAmp, FDA, Opamp..) with noise in the ballpark of non-programmable ones.

 

Finally in case my concerns are true I still need a strategy to achieve  the pre-amplification. My calculations including the whole noise sources of the entire Signal chain indicate that indeed, improving the preamp Input gain is the main Thing to improve Overall Performance. Therefore, 10nV/rtHz would bee substanitally worse than 1 nv/rtHz. Alternatives like switching resistors into resistors into nonprogrammable amps have their own pitfalls too. So what would be the best route to follow to achieve a programmable gainstage with substantially less than ~10 nV/rtHz Input noise?

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