OPA2140AIDR datasheet(16/48 Pages) TI

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7.3.2 Capacitive Load and Stability

The dynamic characteristics of the OPAx140 have been optimized for commonly encountered gains, loads, and

operating conditions. The combination of low closed-loop gain and high capacitive loads decreases the phase

margin of the amplifier and can lead to gain peaking or oscillations. As a result, heavier capacitive loads must be

isolated from the output. The simplest way to achieve this isolation is to add a small resistor (ROUT equal to 50 Ω,

for example) in series with the output.

Small-Signal Overshoot vs Capacitive Load (100-mV Output Step) and Small-Signal Overshoot vs Capacitive

Load (100-mV Output Step) illustrate graphs of Small-Signal Overshoot vs Capacitive Load for several values of

ROUT. Also, see the Feedback Plots Define Op Amp AC Performance Application Bulletin, available for download

from www.ti.com, for details of analysis techniques and application circuits.

7.3.3 Output Current Limit

The output current of the OPAx140 series is limited by internal circuitry to 36 mA/–30 mA (sourcing/sinking),

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

shown in Short Circuit Current vs Temperature.

7.3.4 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

OPA140 and OPA211 are shown with total circuit noise calculated. The op amp itself 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 OPA140, OPA2140, and OPA4140 family 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 OPAx140 series is negligible for any practical source impedance, which makes it

the better choice for applications with high source impedance.

The equation in Figure 7-1 shows the calculation of the total circuit noise, with these parameters:

• en = voltage noise

• In = current noise

• RS = source impedance

• k = Boltzmann's constant = 1.38 × 10–23 J/K

• T = temperature in degrees Kelvin (K)

For more details on calculating noise, see Section 7.3.5.

100k

SourceResistance,R (W)

100

10k

100

+(i R ) +4kTR

ResistorNoise

OPA211

OPA140

Figure 7-1. Noise Performance of the OPA140 and OPA211 in Unity-Gain Buffer Configuration

OPA140, OPA2140, OPA4140

SBOS498E – JULY 2010 – REVISED JULY 2021

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