OPA227 Datasheet, PDF (18/46 Pages) Burr-Brown (TI) – High Precision, Low Noise OPERATIONAL AMPLIFIERS

OPA227, OPA2227, OPA4227
OPA228, OPA2228, OPA4228
SBOS110B – MAY 1998 – REVISED JUNE 2015
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Feature Description (continued)
7.3.5 Input Bias Current Cancellation
The input bias current of the OPAx22x series is internally compensated with an equal and opposite cancellation
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 38) 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 38. Input Bias Current Cancellation
7.3.6 Noise Performance
Figure 39 shows total circuit noise for varying source impedances with the operational amplifier in a unity-gain
configuration (no feedback resistor network, therefore no additional noise contributions). Two different operational
amplifiers are shown with total circuit noise calculated. The OPA227 has very low voltage noise, making it ideal
for low source impedances (less than 20 kΩ). A similar precision operational amplifier, the OPA277, has
somewhat higher voltage noise but lower current noise. It provides excellent noise performance at moderate
source impedance (10 kΩ to 100 kΩ). Above 100 kΩ, a FET-input operational amplifier such as the OPA132
(very low current noise) may provide improved performance. Use the equation in Figure 39 for calculating the
total circuit noise. 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 Basic Noise Calculations.
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