DocID025908 Rev 1

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AN4439

Calculating the input resistor values

15

Figure 4. First stage Op-Amp

Equation 4

Equation 5

Using Kirchoff’s current law (KCL) on the negative input (CSI- or in this equation CSN) we

have the following two equations:

Equation 6

Equation 7

Then solving Equation 6 and Equation 7 for CSN and putting them together obtains:

Equation 8

V

X

R

1

-------

V

OUT1_0V

V

X

–

10R

1

---------------------------------------

=

V

X

V

OUT1_0V

11

-------------------------

=

V

CC

V

CSN

–

R

Bias

--------------------------------

V

CSN

R

IN

---------------

–

V

CSN

V

X

–

11 R

1

⋅

----------------------------

–

V

CSN

V

x

–

R

1

---------------------------

V

x

V

X

–

10 R

1

⋅

--------------------

–

11 V

x

⋅

10

-----------------

V

X

10

-------

–

11 R

⋅⋅

⋅

R

⋅⋅

+

11 R

⋅⋅

11 R

⋅

R

IN

+

R

Bias

⋅⋅

+

--------------------------------------------------------------------------------------------------------------------

=