OPA227 Datasheet, PDF (17/46 Pages) Burr-Brown (TI) – High Precision, Low Noise OPERATIONAL AMPLIFIERS

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OPA227, OPA2227, OPA4227
OPA228, OPA2228, OPA4228
SBOS110B – MAY 1998 – REVISED JUNE 2015
Feature Description (continued)
7.3.3 Offset Voltage Adjustment
The OPAx22x series are laser-trimmed for very low offset and drift so most applications will not require external
adjustment. However, the OPA227 and OPA228 (single versions) provide offset voltage trim connections on pins
1 and 8. Offset voltage can be adjusted by connecting a potentiometer as shown in Figure 36. This adjustment
should be used only to null the offset of the operational amplifier. This adjustment should not be used to
compensate for offsets created elsewhere in the system because this can introduce additional temperature drift.
V+
Trim range exceeds
offset voltage specification
0.1μF
20kΩ
7
2
1
8
OPA227
3
6
4
0.1μF
V–
OPA227 and OPA228 single op amps only.
Use offset adjust pins only to
null offset voltage of op amp.
See text.
Figure 36. OPA227 Offset Voltage Trim Circuit
7.3.4 Input Protection
Back-to-back diodes (see Figure 37) are used for input protection on the OPAx22x. Exceeding the turnon
threshold of these diodes, as in a pulse condition, can cause current to flow through the input protection diodes
due to the amplifier’s finite slew rate. Without external current limiting resistors, the input devices can be
destroyed. Sources of high-input current can cause subtle damage to the amplifier. Although the unit may still be
functional, important parameters such as input offset voltage, drift, and noise may shift.
R_F
500Ω
–
+
Input
OPA227
Output
Figure 37. Pulsed Operation
When using the OPA227 as a unity-gain buffer (follower), the input current should be limited to 20 mA. This can
be accomplished by inserting a feedback resistor or a resistor in series with the source. Use Equation 1 to
calculate sufficient resistor size.
R_X = V_S/20mA – R_SOURCE
where
• R_X is either in series with the source or inserted in the feedback path.
(1)
For example, for a 10-V pulse (V_S = 10 V), total loop resistance must be 500 Ω. If the source impedance is large
enough to sufficiently limit the current on its own, no additional resistors are needed. The size of any external
resistors must be carefully chosen because they will increase noise. See the Noise Performance section of this
data sheet for further information on noise calculation. Figure 37 shows an example implementing a current
limiting feedback resistor.
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Product Folder Links: OPA227 OPA2227 OPA4227 OPA228 OPA2228 OPA4228