OPA2227-EP_13 Datasheet, PDF (10/23 Pages) Texas Instruments – HIGH PRECISION, LOW NOISE OPERATIONAL AMPLIFIER

OPA2227-EP
SBOS594A – MARCH 2012 – REVISED NOVEMBER 2012
www.ti.com
where R_X is either in series with the source or inserted in the feedback path. For example, for a 10-V pulse
(V_S = 10 V), total loop resistance must be 500 Ω. 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 calculation. Figure 2 shows an example implementing a current limiting feedback resistor.
Input Bias Current Cancellation
The input bias current of the OPA2227 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 3) may
actually increase offset and noise and is therefore not recommended.
Conventional Op Amp Configuration
R₂
R₁
Op Amp
R_B = R₂ || R₁
External Cancellation Resistor
Figure 3. Input Bias Current Cancellation
Noise Performance
Figure 4 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 OPA2227 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 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 x 10^–23 J/K and T is temperature in K. For more details on calculating noise, see “Basic Noise Calculations.”
1.00+03
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
E_O
1.00E+02 ^RS
OPA2227
1.00E+01
OPA2227
Resistor Noise
Resistor Noise
1.00E+00
100
E_O2 = e_n2 + (i_n R_S)² + 4kTR_S
1k
10k
100k
1M
Source Resistance, R_S (Ω)
Figure 4. Noise Performance of the OPA2227 in Unity-Gain Buffer Configuration
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