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ADA4941 gain calculations.

Question asked by Garth_Rees on Feb 4, 2013
Latest reply on Feb 5, 2013 by SpiceMan

Trying to do the gain calculations for the ADA4941, which look to be quite simple, BUT.....

I am using a single-ended input, DC-coupled (1.4VDC, mid-rail, single-sided supply 0 - 2.8VDC) with a desired gain of 4, and +/- 1.25V differential input range on a 3VDC single supply ADC. The Vocm is +1.25VDC.

 

OK, the formula's that are given in the datasheet for theVout+ and Vout- voltages are as follows:

 

Vout+ = Vin(1+Rf/Rg) - VG(Rf/Rg)

Vout- = -Vin(1+Rf/Rg) + VG(Rf/Rg) + 2(VREF)

 

The one thing that I cannot find any description of is the value "VG" - what voltage is it? All that is said is that it is an external feed voltage, and from the datasheet drawings, it appears to be the DC bias supply on the -ve input pin - in my case I assume it should be 1.4 volts, to balance the 1.4V DC bias on the +ve input.

 

Any clues or ideas, greatfully received.

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