AN420

Principle of operation of an algebraic adder

Doc ID 2078 Rev 2

5/15

3

Principle of operation of an algebraic adder

Figure 2 represents the generic algebraic adder.

Figure 2.

Generic algebraic adder

The circuit generates an output voltage equal to: i

To minimize the effects of the input polarizing currents, the total resistances seen from the

two inputs of the OpAmp should be the same. Therefore:

The two resistances RP0 and RN0 are needed to satisfy the above relation. In general, only

one of them will be needed.

(1)

(2)

VNn

RNn

Rr

RN2

VN2

VN1

RN1

RN0

Vn

Vo

+

-

Vp

RP1

RP2

VP2

VP1

VPm

RPm

RP0

V

0

K

i

i1

=

m

∑

V

P

i

K

i

i1

=

n

∑

V

N

j

×

–

×

=

1

R

r

------

1

R

N

0

---------

1

R

N

j

--------

j1

=

n

∑

++

1

R

P

0

---------

1

R

P

i

--------

i1

=

m

∑

+

1

R

T

-------

==