8.3 Feature Description

8.3.1 Operating Voltage

The OPAx310 series of operational amplifiers is fully specified from 1.8 V to 5.5 V and ensured for amplifier

operation from 1.5 V to 1.8 V. In addition, many specifications apply from –40°C to 125°C. Parameters that

vary significantly with operating voltages or temperature will be provided in the typical characteristics section at

product release. TI highly recommends to bypass power-supply pins with at least 0.01 μF ceramic capacitors.

8.3.2 Rail-to-Rail Input

The input common-mode voltage range of the OPAx310 series extends to either supply rails. This statement is

true even when operating at the ultra-low supply voltage of 1.5 V, all the way up to the standard supply voltage

of 5.5 V. This performance is achieved with a complementary input stage: an N-channel input differential pair in

parallel with a P-channel differential pair. Refer to the Functional Block Diagram for more details.

For most amplifiers with a complementary input stage, one of the input pairs, usually the P-channel input pair,

is designed to deliver slightly better performance in terms of input offset voltage, offset drift over the N-channel

pair. Consequently, the P-channel pair is designed to cover the majority of the common mode range with the

N-channel pair slated to slowly take over at a certain threshold voltage from the positive rail. Just after the

threshold voltage, both the input pairs are in operation for a small range referred to as the transition region.

Beyond this region, the N-channel pair completely takes over. Within the transition region, PSRR, CMRR, offset

voltage, offset drift, and THD can be degraded compared to device operation outside this region. Hence, most

applications generally prefer operating in the P-channel input range where the performance is slightly better.

For the OPAx310, the P-channel pair is typically active for input voltages from (V–) to (V+) – 0.4 V and the

N-channel pair is typically active for input voltages from the positive supply to (V+) – 0.4 V. The transition region

occurs typically from (V+) – 0.5 V to (V+) – 0.3 V, in which both pairs are on. These voltage levels mentioned

above can vary with process variations associated with threshold voltage of transistors. In the OPAx310, 200 mV

transition region mentioned above can vary up to 200 mV in either direction. Thus, the transition region (both

stages on) can range from (V+) – 0.7 V to (V+) – 0.5 V on the low end, up to (V+) – 0.3 V to (V+) – 0.1 V on the

high end.

8.3.3 Rail-to-Rail Output

Designed as a micro-power, high output current operational amplifier, the OPAx310 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 2 kΩ, the output swings to within a maximum of

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

swing close to the rails.

8.3.4 Capacitive Load and Stability

The OPAx310 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 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 remains stable with a pure capacitive load up to approximately 75 pF with a good phase margin of

45° typical and has no sustained oscillations up to 250 pF. The equivalent series resistance (ESR) of some

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 10 Ω to 20 Ω) in series with the output, as shown in Figure 8-1.

OPA310, OPA2310, OPA4310

SBOSAA1 – APRIL 2022

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