INPUT BIAS CURRENT CANCELLATION
NOISE PERFORMANCE
The input bias current of the OPA227 and OPA228 series is
internally compensated with an equal and opposite cancella-
tion current. The resulting input bias current is the difference
between with input bias current and the cancellation current.
The residual input bias current can be positive or negative.
Figure 4 shows total circuit noise for varying source imped-
ances with the op amp in a unity-gain configuration (no
feedback resistor network, therefore no additional noise con-
tributions). Two different op amps are shown with total circuit
noise calculated. The OPA227 has very low voltage noise,
making it ideal for low source impedances (less than 20kΩ).
Asimilar precision op amp, the OPA277, has somewhat higher
voltage noise but lower current noise. It provides excellent
noise performance at moderate source impedance (10kΩ to
100kΩ). Above 100kΩ, a FET-input op amp such as the
OPA132 (very low current noise) may provide improved
performance. The equation is shown for the calculation of the
total circuit noise. Note that en = voltage noise, in = current
noise, RS = source impedance, k = Boltzmann’s constant =
1.38 • 10–23 J/K and T is temperature in K. For more details on
calculating noise, see the insert titled “Basic Noise Calcula-
tions.”
When the bias current is cancelled in this manner, the input
bias current and input offset current are approximately equal.
A resistor added to cancel the effect of the input bias current
(as shown in Figure 3) may actually increase offset and noise
and is therefore not recommended.
Conventional Op Amp Configuration
R2
R1
Op Amp
Not recommended
for OPA227
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
1.00+03
RB = R2 || R1
External Cancellation Resistor
EO
OPA227
RS
Recommended OPA227 Configuration
1.00E+02
R2
OPA277
OPA277
R1
Resistor Noise
OPA227
1.00E+01
1.00E+00
Resistor Noise
EO2 = en2 + (in RS)2 + 4kTRS
OPA227
No cancellation resistor.
See text.
100
1k
10k
100k
10M
Source Resistance, RS (Ω)
FIGURE 4. Noise Performance of the OPA227 in Unity-
Gain Buffer Configuration.
FIGURE 3. Input Bias Current Cancellation.
BASIC NOISE CALCULATIONS
noise component. The voltage noise is commonly mod-
Design of low noise op amp circuits requires careful
consideration of a variety of possible noise contributors:
noise from the signal source, noise generated in the op
amp, and noise from the feedback network resistors. The
total noise of the circuit is the root-sum-square combina-
tion of all noise components.
eled as a time-varying component of the offset voltage.
The current noise is modeled as the time-varying compo-
nent of the input bias current and reacts with the source
resistance to create a voltage component of noise. Conse-
quently, the lowest noise op amp for a given application
depends on the source impedance. For low source imped-
ance, current noise is negligible and voltage noise gener-
ally dominates. For high source impedance, current noise
may dominate.
The resistive portion of the source impedance produces
thermal noise proportional to the square root of the
resistance. This function is shown plotted in Figure 4.
Since the source impedance is usually fixed, select the op
amp and the feedback resistors to minimize their contri-
bution to the total noise.
Figure 5 shows both inverting and noninverting op amp
circuit configurations with gain. In circuit configurations
with gain, the feedback network resistors also contribute
noise. The current noise of the op amp reacts with the
feedback resistors to create additional noise components.
The feedback resistor values can generally be chosen to
make these noise sources negligible. The equations for
total noise are shown for both configurations.
Figure 4 shows total noise for varying source imped-
ances with the op amp in a unity-gain configuration (no
feedback resistor network and therefore no additional
noise contributions). The operational amplifier itself con-
tributes both a voltage noise component and a current
OPA227, 2227, 4227
OPA228, 2228, 4228
12
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