OPA277, OPA2277, OPA4277 9
SBOS079A www.ti.com
Op Amp
(a)
OPA277
(b)
No bias current
cancellation resistor
(see text)
Conventional op amp with external bias
current cancellation resistor. OPA277 with no external bias current
cancellation resistor.
R
2
R
1
R
2
R
1
R
B
= R
2
|| R
1
APPLICATIONS INFORMATION
The OPA277 series is unity-gain stable and free from unex-
pected output phase reversal, making it easy to use in a wide
range of applications. Applications with noisy or high imped-
ance power supplies may require decoupling capacitors
close to the device pins. In most cases 0.1µF capacitors are
adequate.
The OPA277 series has very low offset voltage and drift. To
achieve highest performance, circuit layout and mechanical
conditions should be optimized. Offset voltage and drift can
be degraded by small thermoelectric potentials at the op amp
inputs. Connections of dissimilar metals will generate thermal
potential which can degrade the ultimate performance of the
OPA277 series. These thermal potentials can be made to
cancel by assuring that they are equal in both input terminals.
•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
cooling fans.
OPERATING VOLTAGE
OPA277 series op amp operate from ±2V to ±18V supplies
with excellent performance. Unlike most op amps which are
specified at only one supply voltage, the OPA277 series is
specified for real-world applications; a single limit applies
over the ±5V to ±15V supply range. This allows a customer
operating at VS= ±10V to have the same assured perfor-
mance as a customer using ±15V supplies. In addition, key
parameters are assured over the specified temperature range,
–40°C to +85°C. Most behavior remains unchanged through
the full operating voltage range (±2V to ±18V). Parameters
which vary significantly with operating voltage or temperature
are shown in typical performance curves.
OFFSET VOLTAGE ADJUSTMENT
The OPA277 series is laser-trimmed for very low offset
voltage and drift so most circuits will 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 1. This
adjustment should be used only to null the offset of the op
amp. This adjustment should not be used to compensate for
offsets created elsewhere in a system since this can intro-
duce additional temperature drift.
INPUT PROTECTION
The inputs of the OPA277 series are protected with 1kΩ
series input resistors and diode clamps. The inputs can
withstand ±30V differential inputs without damage. The pro-
tection diodes will, of course, conduct current when the
inputs are over-driven. This may disturb the slewing behavior
of unity-gain follower applications, but will not damage the op
amp.
INPUT BIAS CURRENT CANCELLATION
The input stage base current of the OPA277 series is
internally compensated with an equal and opposite cancella-
tion 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 op
amps (Figure 2). A resistor added to cancel input bias current
errors may actually increase offset voltage and noise.
FIGURE 2. Input Bias Current Cancellation.
V+
V–
20kΩ
OPA277 single op amp only.
Use offset adjust pins only to null
offset voltage of op amp—see text.
Trim Range: Exceeds
Offset Voltage Specification
OPA277 6
7
8
4
3
21
0.1µF
0.1µF
FIGURE 1. OPA277 Offset Voltage Trim Circuit.
