12

FN7490.4

February 14, 2008

connected to GND. With this configuration, Equation 1 is only

true for a positive swing in VIN; negative input swings will be

ignored and the output will be at ground.

Reference Connection

Unlike a three op amp instrumentation amplifier, a finite

series resistance seen at the REF terminal does not degrade

the EL8170 and EL8173's high CMRR performance,

eliminating the need for an additional external buffer

amplifier. The circuit shown in Figure 40 uses the FB+ pin as

a REF terminal to center or to adjust the output. Because the

FB+ pin is a high impedance input, an economical resistor

divider can be used to set the voltage at the REF terminal.

The reference voltage error due to the input bias current is

minimized by keeping the values of the voltage divider

Equation 2. Note that any noise or unwanted signals on the

reference supply will be amplified at the output according to

Equation 2.

The FB+ pin can also be connected to the other end of

the EL8170 and EL8173 in-amps maintain constant

differential voltage across the input terminals and feedback

terminals (IN+ - IN- = FB+ - FB-), the transfer function of

gain term is eliminated, and susceptibility to external noise is

reduced.

1

3

2

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

V+

V-

EN

RF

RG

VOUT

EN

EL8170/3

VCM

VIN/2

VIN/2

FIGURE 39. GAIN IS SET BY TWO EXTERNAL RESISTORS,

V

OUT

1

R

F

R

G

--------

+

⎝⎠

⎜⎟

⎛⎞

V

IN

=

(EQ. 1)

V

OUT

1

R

F

R

G

--------

+

⎝⎠

⎜⎟

⎛⎞

V

IN

()

1

R

F

R

G

--------

+

⎝⎠

⎜⎟

⎛⎞

V

REF

()

+

=

(EQ. 2)

FIGURE 40. GAIN SETTING AND REFERENCE CONNECTION

1

3

2

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

V+

V-

EN

+2.4V TO +5.5V

RF

RG

VOUT

EN

EL8170,

VCM

VIN/2

VIN/2

REF

+2.4V TO +5.5V

R1

R2

EL8173

FIGURE 41. REFERENCE CONNECTION WITH AN AVAILABLE

1

3

2

8

5

4

7

6

+

-

+

-

IN+

IN-

FB+

FB-

V+

V-

EN

RF

RG

VOUT

EN

EL8170,

VCM

VIN/2

VIN/2

VREF

+2.4V TO +5.5V

EL8173

V

OUT

1

R

F

R

G

--------

+

⎝⎠

⎜⎟

⎛⎞

V

IN

()

V

REF

()

+

=

(EQ. 3)

EL8170, EL8173