OPA2227MDREP datasheet(10/23 Pages) TI1

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VOLTAGE NOISE SPECTRAL DENSITY

vs SOURCE RESISTANCE

100k

Source Resistance, R

S (Ω)

100

10k

1.00+03

1.00E+02

1.00E+01

1.00E+00

OPA2227

Resistor Noise

OPA2227

2 = e

2 + (i

n RS)

2 + 4kTR

Op Amp

B = R2 || R1

External Cancellation Resistor

Conventional Op Amp Configuration

OPA2227-EP

SBOS594A – MARCH 2012 – REVISED NOVEMBER 2012

www.ti.com

where RX is either in series with the source or inserted in the feedback path. For example, for a 10-V pulse

(VS = 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.

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 k

Ω). 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 k

Ω 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 en = voltage noise, in = current noise, RS = 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.”

Figure 4. Noise Performance of the OPA2227 in Unity-Gain Buffer Configuration

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