APPLICATIONS INFORMATION
Trim range exceeds
offset voltage specification
V+
The OPA227 and OPA228 series are precision op amps with
very low noise. The OPA227 series is unity-gain stable with
a slew rate of 2.3V/µs and 8MHz bandwidth. The OPA228
series is optimized for higher-speed applications with gains
of 5 or greater, featuring a slew rate of 10V/µs and 33MHz
bandwidth. Applications with noisy or high impedance
power supplies may require decoupling capacitors close to
the device pins. In most cases, 0.1µF capacitors are ad-
equate.
0.1µF
20kΩ
7
1
2
3
8
OPA227
6
OPA227 and OPA228 single op amps only.
Use offset adjust pins only to
null offset voltage of op amp.
See text.
4
0.1µF
V–
OFFSET VOLTAGE AND DRIFT
The OPA227 and OPA228 series have very low offset
voltage and drift. To achieve highest dc precision, circuit
layout and mechanical conditions should be optimized.
Connections of dissimilar metals can generate thermal po-
tentials at the op amp inputs which can degrade the offset
voltage and drift. These thermocouple effects can exceed
the inherent drift of the amplifier and ultimately degrade its
performance. The thermal potentials can be made to cancel
by assuring that they are equal at both input terminals. In
addition:
FIGURE 1. OPA227 Offset Voltage Trim Circuit.
amp. This adjustment should not be used to compensate for
offsets created elsewhere in the system since this can
introduce additional temperature drift.
INPUT PROTECTION
Back-to-back diodes (see Figure 2) are used for input protec-
tion on the OPA227 and OPA228. Exceeding the turn-on
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 param-
eters such as input offset voltage, drift, and noise may shift.
• Keep thermal mass of the connections made to the two
input terminals similar.
• Locate heat sources as far as possible from the critical
input circuitry.
• Shield op amp and input circuitry from air currents such
as those created by cooling fans.
RF
500Ω
OPERATING VOLTAGE
OPA227 and OPA228 series op amps operate from ±2.5V to
±18V supplies with excellent performance. Unlike most op
amps which are specified at only one supply voltage, the
OPA227 series is specified for real-world applications; a
single set of specifications applies over the ±5V to ±15V
supply range. Specifications are guaranteed for applications
between ±5V and ±15V power supplies. Some applications
do not require equal positive and negative output voltage
swing. Power supply voltages do not need to be equal. The
OPA227 and OPA228 series can operate with as little as 5V
between the supplies and with up to 36V between the
supplies. For example, the positive supply could be set to
25V with the negative supply at –5V or vice-versa. In
addition, key parameters are guaranteed over the specified
temperature range, –40°C to +85°C. Parameters which vary
significantly with operating voltage or temperature are shown
in the Typical Performance Curves.
–
OPA227
Output
+
Input
FIGURE 2. Pulsed Operation.
When using the OPA227 as a unity-gain buffer (follower), the
input current should be limited to 20mA. This can be accom-
plished by inserting a feedback resistor or a resistor in series
with the source. Sufficient resistor size can be calculated:
RX = VS/20mA – RSOURCE
where RX is either in series with the source or inserted in
the feedback path. For example, for a 10V pulse (VS =
10V), total loop resistance must be 500Ω. 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 since they
will increase noise. See the Noise Performance section of
this data sheet for further information on noise calcula-
tion. Figure 2 shows an example implementing a current-
limiting feedback resistor.
OFFSET VOLTAGE ADJUSTMENT
The OPA227 and OPA228 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 con-
nections on pins 1 and 8. Offset voltage can be adjusted by
connecting a potentiometer as shown in Figure 1. This
adjustment should be used only to null the offset of the op
®
OPA227, 2227, 4227
OPA228, 2228, 4228
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