TL05X datasheet(43/67 Pages) STMICROELECTRONICS

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TL05x, TL05xA

ENHANCED-JFET LOW-OFFSET

OPERATIONAL AMPLIFIERS

SLOS178A – FEBRUARY 1997 - REVISED FEBRUARY 2003

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

instrumentation amplifier with adjustable gain/null

The instrumentation amplifier in Figure 86 benefits greatly from the high input impedance and stable input offset

voltage of the TL05xA. Amplifiers U1A, U1B, and U2A form the actual instrumentation amplifier, while U2B

provides offset null. Potentiometer R1 provides gain adjustment. With R1 = 2 k

Ω, the circuit gain equals 100,

while with R1 = 200 k

Ω, the circuit gain equals two. The following equation shows the instrumentation amplifier

gain as a function of R1:

+ 1 ) R2 ) R3

Readjusting the offset null is necessary when the circuit gain is changed. If U2B is needed for another

application, R7 can be terminated at ground. The low input offset voltage of the TL05xA minimizes the dc error

of the circuit. For best matching, all resistors should be one-percent tolerance. The matching between R4, R5,

R6, and R7 controls the CMRR of this application.

The following equation shows the output voltages when the input voltage equals zero. This dc error can be

nulled by adjusting the offset null potentiometer; however, any change in offset voltage over time or temperature

also creates an error. To calculate the error from changes in offset, consider the three offset components in the

equation as delta offsets, rather than initial offsets. The improved stability of Texas Instruments enhanced JFETs

minimizes the error resulting from change in input offset voltage with time. Assuming VI equals zero, VO can

be shown as a function of the offset voltage:

–V

IO1

) R7

) R6

) R2

) V

IO3

) R6

O +

IO2

) R3

) R7

) R6

) R2

NOTE A: U1 and U2 = TL05xA; VCC

± = ±15 V.

100 k

Ω

U2A

–

VI–

U1A

10 k

Ω

10 k

Ω

200 k

Ω

10 M

Ω

100 k

Ω

10 turn

AV = 2 to 100

2 k

Ω

U1B

VI+

U2B

0.1

µF

Offset Null

VCC–

82 k

Ω

82 k

Ω

VCC+

10 k

Ω

10 k

Ω

10 M

Ω

1 k

Ω

Figure 86. Instrumentation Amplifier

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