7.3.1 Operating Characteristics

The OPAx377-Q1 family of amplifiers has parameters that are fully specified from 2.2 V to 5.5 V (±1.1 V to ±2.75 V). Many of the specifications apply from –40°C to +125°C. Parameters that can exhibit significant variance with regard to operating voltage or temperature are presented in the Typical Characteristics section.

7.3.2 Common-Mode Voltage Range

The input common-mode voltage range of the OPAx377-Q1 series extends 100 mV beyond the supply rails. The offset voltage of the amplifier is low, from approximately (V–) to (V+) – 1 V, as shown in Figure 22. The offset voltage increases as common-mode voltage exceeds (V+) – 1 V. Common-mode rejection is specified from (V–) to (V+) – 1.3 V.

Figure 22. Offset and Common-Mode Voltage

7.3.3 Input and ESD Protection

The OPAx377-Q1 family incorporates internal electrostatic discharge (ESD) protection circuits on all pins. In the case of input and output pins, this protection primarily consists of current steering diodes connected between the input and power-supply pins. These ESD protection diodes also provide in-circuit, input overdrive protection, as long as the current is limited to 10 mA as stated in the Absolute Maximum Ratings table.

Figure 23 shows how a series input resistor may be added to the driven input to limit the input current. The added resistor contributes thermal noise at the amplifier input and its value must be kept to a minimum in noise-sensitive applications.

Figure 23. Input Current Protection

7.3.4 EMI Susceptibility and Input Filtering

Operational amplifiers vary in susceptibility to electromagnetic interference (EMI). If conducted EMI enters the operational amplifier, the dc offset observed at the amplifier output may shift from the nominal value while the EMI is present. This shift is a result of signal rectification associated with the internal semiconductor junctions. While all amplifier pin functions can be affected by EMI, the input pins are likely to be the most susceptible. The OPAx377-Q1 operational amplifier family incorporates an internal input low-pass filter that reduces the amplifier response to EMI. Both common-mode and differential mode filtering are provided by the input filter. The filter is designed for a cutoff frequency of approximately 75 MHz (–3 dB), with a roll-off of 20 dB per decade.

7.3.5 Capacitive Load and Stability

The OPAx377-Q1 series of amplifiers may be used in applications where driving a capacitive load is required. As with all op amps, there may be specific instances where the OPAx377-Q1 can become unstable, leading to oscillation. The particular op amp circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing whether an amplifier will be stable in operation. An op amp in the unity-gain (1 V/V) buffer configuration and driving 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 op amp output resistance, creates a pole within the feedback loop that degrades the phase margin. The degradation of the phase margin increases as the capacitive loading increases.

The OPAx377-Q1 in a unity-gain configuration can directly drive up to 250-pF pure capacitive load. Increasing the gain enhances the ability of the amplifier to drive greater capacitive loads; see the typical characteristic plot, Figure 15. In unity-gain configurations, capacitive load drive can be improved by inserting a small (10-Ω to 20-Ω) resistor, R_S, in series with the output, as shown in Figure 24. This resistor significantly reduces ringing while maintaining dc performance for purely capacitive loads. However, if there is a resistive load in parallel with the capacitive load, a voltage divider is created, introducing a gain error at the output and slightly reducing the output swing. The error introduced is proportional to the ratio R_S/R_L, and is generally negligible at low output current levels.

Figure 24. Improving Capacitive Load Drive