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TL054MFKB

ENHANCED-JFET LOW-OFFSET OPERATIONAL AMPLIFIERS

STMicroelectronics 

TL05x, TL05xA

ENHANCED-JFET LOW-OFFSET

OPERATIONAL AMPLIFIERS

SLOS178A – FEBRUARY 1997 - REVISED FEBRUARY 2003

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 kΩ, the circuit gain equals 100,

while with R1 = 200 kΩ, the circuit gain equals two. The following equation shows the instrumentation amplifier

+ ) ǒ ) Ǔ gain as a function of R1:

AV 1

R2 R3

R1

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:

ƪǒ Ǔ ǒ Ǔ ǒ Ǔ ǒ Ǔƫ + ) ) ) ) VO

VIO2 1

R3

R1

R7

R5 R7

1

R6

R4

R2 R6

R1 R4

ƪ ǒ Ǔ ǒ Ǔ ǒ Ǔƫ ǒ Ǔ ) ) ) ) ) ) –VIO1

R3

R1

R7

R5 R7

1

R6

R4

R6

R4

1

R2

R1

VIO3 1

R6

R4

VI– +

U1A

–

100 kΩ

R2

200 kΩ

10 turn

AV = 2 to 100

2 kΩ R1

10 MΩ

10 MΩ

R3

100 kΩ

U1B

VI+

R4

10 kΩ

R6

10 kΩ

U2A

R5

10 kΩ

R7

10 kΩ

VO

VCC+

U2B

Offset Null

0.1 µF

82 kΩ

1 kΩ

82 kΩ

NOTE A: U1 and U2 = TL05xA; VCC± = ±15 V.

Figure 86. Instrumentation Amplifier

VCC–

• POST OFFICE BOX 655303 DALLAS, TEXAS 75265

43

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