[SOLVED] AD628 ic gain

[ats2]

Hello,
Can anybody help me about the total gain of below circuit?
I know it is basic, but am new at analog stuff.
I search on web, and saw nearly similar circuits, but Rext3 confused me
Thank you.

 

[FvM]

The purpose of Rext3 is explained in the datasheet on the following page. It has no effect on gain.

At a 2 nA maximum, the input bias current of the buffer amplifier is very low and any offset voltage induced at the buffer amplifier by its bias current may be neglected (2 nA × 10 kΩ = 20 μV). However, to absolutely minimize bias current effects, select REXT1 and REXT2 so that their parallel combination is 10 kΩ. If practical resistor values force the parallel combination of REXT1 and REXT2 below 10 kΩ, add a series resistor (REXT3) to make up for the difference.

 

[KlausST]

Hi,

A = 0.1 x (1 + REXT1/REXT2)

Klaus

 

[ats2]

Hi,

A = 0.1 x (1 + REXT1/REXT2)

Klaus

Thank you, What about reference voltage?

 

[KlausST]

Hi,

The reference voltage has nothing to do with gain.

Klaus

 

[ats2]

Thank you Klaus. Wyh adding then subtracting the Vref voltage?

 

[FvM]

Vout = Vref + A *(Vin+ - Vin-)

 

[ats2]

Thank you, the second stage opamp resistor connected to vref, instead of gnd. Is it not a subtraction?
I guess in first stage opamp adds Vref voltage to Vin+ as you explain.

 

[KlausST]

Hi,

Maybe you misunderstand the circuit, maybe you misunderstand Vref.

The circuit just references it's output voltage to a certain voltage (Vref) instead of GND.

This "Vref" has nothing to do with Vref of an ADC or DAC.

****
Example:
Let's say you have an AC current measurement shunt. With a voltage across it of +/-0.1V peak.
The resistor is connected at a node with variable voltage.
(It could be a speaker current measurement at a BTL audio amplifier)
And you have an ADC with unipolar input voltage range of 0V ... 3.0V.
Now to be able to measure the AC voltage you need to add 1.5V at the amplifier output.
This 1.5V is the Vref of your amplifier.

And to use the full range of your ADC you need to amplify your signal with a gain of 15.

Your circuit/amplifier can fulfill both requiremens. Adding gain (multiply) and adding offset

Klaus