7.3.2 Output Current Limit

The output current of the OPAx145 is limited by internal circuitry to +20 mA (sinking) and –20 mA (sourcing) to

protect the device if the output is accidentally shorted. This short-circuit current depends on temperature, as

shown in Figure 6-36.

7.3.3 Noise Performance

Figure 7-1 shows the total circuit noise for varying source impedances with the operational amplifier in a unity-

gain configuration (with no feedback resistor network and therefore no additional noise contributions). The

OPAx145 and OPAx211 are shown with total circuit noise calculated. The op amp contributes both a voltage

noise component and a current noise component. The voltage noise is commonly modeled as a time-varying

component of the offset voltage. The current noise is modeled as the time-varying component of the input bias

current and reacts with the source resistance to create a voltage component of noise. Therefore, the lowest

noise op amp for a given application depends on the source impedance. For low source impedance, current

noise is negligible, and voltage noise generally dominates. The OPAx145 has both low voltage noise and

extremely low current noise because of the FET input of the op amp. As a result, the current noise contribution of

the OPAx145 is negligible for any practical source impedance, which makes it the better choice for applications

with high source impedance.

0.1n

1n

10n

100n

1µ

10µ

1

10

100

1k

10k

100k

1M

10M

Source Resistance, R

C003

OPA211

OPA145

Resistor Noise

R

Figure 7-1.

Figure 7-1. Noise Performance of the OPAx145 and OPA211 in Unity-Gain Buffer Configuration

Equation 1 can be used to calculate the total noise at the output of the amplifier. A plot can be created using this

equation to quickly compare the noise performance of two different amplifiers when used with different source

resistances, as is shown in Figure 7-1.

u

2

2

2

O

n

n

S

S

E

e

(i

R )

4kTR

(1)

where:

• T = temperature in kelvins (K)

For more details on calculating noise, see Section 7.3.4.

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OPA145,, OPA2145

SBOS427E – JUNE 2017 – REVISED OCTOBER 2020

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