Input Common-Mode Voltage (V)

-3 -2.5

-2 -1.5

-1 -0.5

0

0.5

1

1.5

2

2.5

3

-1600

-1200

-800

-400

0

400

800

1200

1600

D07_

V+ = 2.75V, V– = –2.75V

Figure 6-1. OPAx310-Q1 Offset Voltage vs

Common-Mode

Common-Mode Voltage (V)

-4

-3

-2

-1

0

1

2

3

4

-2000

-1500

-1000

-500

0

500

1000

1500

2000

D004

V+ = 2.75V, V– = –2.75V

Figure 6-2. TLV900x Offset Voltage vs Common-

Mode

6.3.3 Rail-to-Rail Output

Designed as a micro-power, high output current operational amplifier, the OPAx310-Q1 delivers a robust output

drive capability. A class AB output stage with common-source transistors is used to achieve full rail-to-rail output

swing capability. At room temperature and for resistive loads up to 2kΩ, the output swings to within a maximum

of 20mV of either supply rail at 5.5V power supply. Different load conditions change the ability of the amplifier to

swing close to the rails.

6.3.4 Capacitive Load and Stability

The OPAx310-Q1 is designed to be used in applications where driving a capacitive load is required. As with

all operational amplifiers, there can be specific instances where the OPAx310-Q1 can become unstable. The

particular operational amplifier circuit configuration, layout, gain, and output loading are some of the factors to

consider when establishing whether or not an amplifier is stable in operation. An operational amplifier in the

unity-gain (1 V/V) buffer configuration that drives a capacitive load exhibits a greater tendency to be unstable

than an amplifier operated at a higher noise gain. The capacitive load, in conjunction with the operational

amplifier output resistance, creates a pole within the feedback loop that degrades the phase margin. The

degradation of the phase margin increases when capacitive loading increases. When operating in the unity-gain

configuration, the OPAx310-Q1 remains stable with a pure capacitive load up to approximately 75pF with a good

phase margin of 40° typical and has no sustained oscillations up to 250pF. The equivalent series resistance

(ESR) of some very large capacitors (CL greater than 1μF) is sufficient to alter the phase characteristics in

the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop gain allows the

amplifier to drive increasingly larger capacitance. This increased capability is evident when measuring the

overshoot response of the amplifier at higher voltage gains.

One technique for increasing the capacitive load drive capability of the amplifier operating in a unity-gain

configuration is to insert a small resistor (typically 10Ω to 20Ω) in series with the output, as shown in Figure 6-3.

This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. One possible

problem with this technique, however, is that a voltage divider is created with the added series resistor and any

resistor connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output

that reduces the output swing.

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OPA310-Q1

SBOSAI0A – DECEMBER 2023 – REVISED MAY 2024

Copyright © 2024 Texas Instruments Incorporated

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