OPA2277-EP Datasheet, PDF (13/21 Pages) Texas Instruments – OPA2277-EP High-Precision, Low-Noise Operational Amplifier

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OPA2277-EP
SBOS700 – DECEMBER 2014
9.2.2 Detailed Design Procedure
9.2.2.1 Offset Voltage Adjustment
The OPA2277 is laser-trimmed for very-low offset voltage and drift so most circuits do not require external
adjustment. However, offset voltage trim connections are provided on pins 1 and 8. Offset voltage can be
adjusted by connecting a potentiometer as shown in Figure 22. Only use this adjustment to null the offset of the
operational amplifier. Do not use this adjustment to compensate for offsets created elsewhere in a system
because this can introduce additional temperature drift.
V+
0.1 μF
Trim Range: Exceeds
Offset Voltage Specification
20 kΩ
7
21
8
OPA2277
3
6
0.1 μF ⁴
OPA2277 single op amp only.
Use offset adjust pins only to null
offset voltage of op amp—see text.
V–
Figure 22. OPA2277 Offset Voltage Trim Circuit
9.2.2.2 Input Protection
The inputs of the OPA2277 are protected with 1-kΩ series input resistors and diode clamps. The inputs can
withstand ±30-V differential inputs without damage. The protection diodes conduct current when the inputs are
overdriven. This may disturb the slewing behavior of unity-gain follower applications, but does not damage the
operational amplifier.
9.2.2.3 Input Bias Current Cancellation
The input stage base current of the OPA2277 is internally compensated with an equal and opposite cancellation
circuit. The resulting input bias current is the difference between the input stage base current and the
cancellation current. This residual input bias current can be positive or negative.
When the bias current is canceled in this manner, the input bias current and input offset current are
approximately the same magnitude. As a result, it is not necessary to use a bias current cancellation resistor as
is often done with other operational amplifiers (see Figure 23). A resistor added to cancel input bias current
errors may actually increase offset voltage and noise.
R₂
R₂
R₁
R₁
Op Amp
OPA2277
R_B = R₂ || R₁
No bias current
cancellation resistor
(see text)
(a)
Conventional operational amplifier with external
bias current cancellation resistor.
(b)
OPA2277 with no external bias current
cancellation resistor.
Figure 23. Input Bias Current Cancellation
Copyright © 2014, Texas Instruments Incorporated
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