OPA227_07 Datasheet, PDF (12/27 Pages) Texas Instruments – High Precision, Low Noise OPERATIONAL AMPLIFIERS

INPUT BIAS CURRENT CANCELLATION
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.
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
Not recommended
for OPA227
Op Amp
R_B = R₂ || R₁
External Cancellation Resistor
Recommended OPA227 Configuration
R₂
R₁
OPA227
No cancellation resistor.
See text.
FIGURE 3. Input Bias Current Cancellation.
NOISE PERFORMANCE
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).
A similar 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 e_n = voltage noise, i_n = current
noise, R_S = 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.”
1.00+03
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
E_O
1.00E+02 ^RS
OPA227
1.00E+01
OPA227
OPA277
OPA277
Resistor Noise
Resistor Noise
1.00E+00
100
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
10M
Source Resistance, R_S (Ω)
FIGURE 4. Noise Performance of the OPA227 in Unity-
Gain Buffer Configuration.
BASIC NOISE CALCULATIONS
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.
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 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
noise component. The voltage noise is commonly mod-
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.
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.
OPA227, 2227, 4227
12
OPA228, 2228, 4228
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