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Need high gain (Av=1000) and low Vos

Question asked by marty@infinite on Nov 4, 2011
Latest reply on Nov 22, 2011 by OliverSedlacek

I am designing a circuit to amplify the voltage output of a sensor with a low source impedance and then read that voltage with a 16-bit ADC.  The nominal sensor output will be in the 1mV range, changes will be in the 0.1mV range, and its source impedance is under 1000 Ohms.

 

I wish to amplify the voltage output by +1000, and need an input voltage offset of 10uV or less.  I also need the 1/f noise to be low.  I am considering the ADA4528-1 for this application, but there is a discrepancy in the specs.  The numerical spec for the open-loop voltage gain is 130dB, but the graph (Fig 25) on page 9 of the spec sheet shows the open-loop gain flattening out at about 70dB.  If the numerical spec is correct, I believe that this will be an appropriate device for this app.  However, if the graph is correct, I will only have 10dB of feedback, which is too low for comfort.

 

Question 1:  Does anyone know which spec is correct (or whether I am misunderstanding the specs)?

Question 2:  Does anyone see a problem with measuring a 1mV DC signal accurately with a gain of +1000 (aside from the obvious problems of noise pick up, bandwidth limiting, etc.)?

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