EL8172(2007) 查看數據表（PDF） - Intersil

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EL8172
(Rev.:2007)

Micropower, Single Supply, Rail-to-Rail Input-Output Instrumentation Amplifiers

Intersil 

EL8171, EL8172

VIN/2

VIN/2

VCM

2.4V TO 5.5V

R1

REF

R2

RG

2.4V TO 5.5V

71

2 IN+

V+ EN

+

3 IN-

-

8 FB+ EL8171/2

6

+

5 FB- -

V-

4

RF

EN

VOUT

FIGURE 40. CIRCUIT 2 - GAIN SETTING AND REFERENCE

CONNECTION

VOUT

=

⎛

⎜1

⎝

+

R-R----G-F--⎠⎟⎞

(

VIN

)

+

⎛

⎜

⎝

1

+

R-R----G-F--⎠⎟⎞

(

VRE

F

)

(EQ. 2)

The FB+ pin is used 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 without degrading or affecting the CMRR

performance. Any voltage applied to the REF terminal will

shift VOUT by VREF times the closed loop gain, which is set

by resistors RF and RG. See Circuit 2 (Figure 40). 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 resistor,

RG. See Circuit 3 (Figure 41). Keeping the basic concept that

the EL8171 and EL8172 in-amps maintain constant differential

voltage across the input terminals and feedback terminals (IN+

- IN- = FB+ - FB-), the transfer function of Circuit 3 can be

derived. Note that the VREF gain term is eliminated and

susceptibility to external noise is reduced, however the VREF

source must be capable of sourcing or sinking the feedback

current from VOUT through RF and RG.

VIN/2

VCM

VIN/2

2.4V TO 5.5V

71

2 IN+

V+ EN

+

3 IN-

-

8 FB+ EL8171/2

6

+

5 FB- -

V-

4

EN

VOUT

External Resistor Mismatches

Because of the independent pair of feedback terminals

provided by the EL8171 and EL8172, the CMRR is not

degraded by any resistor mismatches. Hence, unlike a three op

amp and especially a two op amp in-amp, the EL8171 and

EL8172 reduce the cost of external components by allowing the

use of 1% or more tolerance resistors without sacrificing CMRR

performance. The EL8171 and EL8172 CMRR will be

maintained regardless of the tolerance of the resistors used.

Gain Error and Accuracy

The EL8172 has a Gain Error (EG) of 0.2% typical. The

EL8171 has an EG of 0.15% typical. The gain error indicated

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

gain error of the EL8171 and EL8172 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:

VOUT

=

⎛

⎜1

⎝

+

R-R----G-F--⎠⎟⎞

× [1 – (ERG + ERF + EG)] × VIN

(EQ. 4)

Where:

ERG = Tolerance of RG

ERF = Tolerance of RF

EG = Gain Error of the EL8171 or EL8172

The term [1-(ERG +ERF +EG)] is the deviation from the

theoretical gain. Thus, (ERG +ERF +EG) is the total gain

error. For example, if 1% resistors are used for the EL8171,

the total gain error would be:

= ±(ERG + ERF + EG(typical))

= ±(0.01 + 0.01 + 0.003)

(EQ. 5)

= ±2.3%

Disable/Power-Down

The EL8171 and EL8172 can be powered down reducing

the supply current to typically 4.5µA. 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 EN bar is pulled above 2V, and will power on when EN

bar is pulled below 0.8V.

RG

RF

VREF

FIGURE 41. CIRCUIT 3 - REFERENCE CONNECTION WITH AN

AVAILABLE VREF

VOUT

=

⎛

⎜1

⎝

+

R-R----G-F--⎠⎟⎞

(

VIN

)

+

(

VREF

)

(EQ. 3)

12

FN6293.3

August 3, 2007

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