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FN7490.4

February 14, 2008

External Resistor Mismatches

Because of the independent pair of feedback terminals

provided by the EL8170 and EL8173, the CMRR is not

degraded by any resistor mismatches. Hence, unlike a three

op amp and especially a two op amp in-amp, the EL8170

and EL8173 reduce the cost of external components by

allowing the use of 1% or more tolerance resistors without

sacrificing CMRR performance. The EL8170 and EL8173

CMRR is maintained regardless of the tolerance of the

resistors used.

Gain Error and Accuracy

in the “Electrical Specifications” table on page 2 is the

inherent gain error of the EL8170 and EL8173 and does not

include the gain error contributed by the resistors. There is

an additional gain error due to the tolerance of the resistors

used. The resulting non-ideal transfer function effectively

becomes Equation 4:

Where:

error. For example, if 1% resistors are used for the EL8170,

the total gain error would be:

Disable/Power-Down

The EL8170 and EL8173 can be powered down reducing

the supply current to typically 2.9µA. When disabled, the

output is in a high impedance state. The active low EN bar

pin has an internal pull-down and hence can be left floating

and the in-amp enabled by default. When the EN bar is

connected to an external logic, the in-amp will power down

when the EN bar is pulled above 2V, and will power-on when

the EN bar is pulled below 0.8V.

Power Dissipation

It is possible to exceed the +150°C maximum junction

temperatures under certain load and power-supply

conditions. It is therefore important to calculate the

to determine if power supply voltages, load conditions, or

package type need to be modified to remain in the safe

operating area. These parameters are related in Equation 6:

where:

Equation 7:

where:

•

application

V

OUT

1

R

F

R

G

--------

+

⎝⎠

⎜⎟

⎛⎞

1E

RG

E

RF

E

G

++

()

–

[] V

IN

×

×

=

(EQ. 4)

E

RG

E

RF

E

()

++

()

±

=

0.01

0.01

0.003

++

()

±

=

2.3%

±

=

(EQ. 5)

T

JMAX

T

MAX

θ

()

+

=

(EQ. 6)

PD

MAX

2*V

S

I

SMAX

V

S

(

- V

V

OUTMAX

R

L

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

×

+

×

=

(EQ. 7)

EL8170, EL8173