7.3.1 Operating Voltage

The OPAx314-Q1 op-amp family is fully specified and ensured for operation from 1.8 V to 5.5 V. In addition, many specifications apply from –40°C to +125°C. Parameters that vary significantly with operating voltages or temperature are shown in the Typical Characteristics section. Power-supply pins must be bypassed with 0.01-μF ceramic capacitors.

7.3.2 Rail-to-Rail Input

The input common-mode voltage range of the OPAx314-Q1 family extends 200 mV beyond the supply rails. This performance is achieved with a complementary input stage: an N-channel input differential pair in parallel with a P-channel differential pair. The N-channel pair is active for input voltages close to the positive rail, typically (V+) – 1.3 V to 200 mV above the positive supply. The P-channel pair is on for inputs from 200 mV below the negative supply to approximately (V+) – 1.3 V. A small transition region exists, typically (V+) – 1.4 V to (V+)
– 1.2 V, in which both pairs are on. This 200-mV transition region can vary up to 300 mV with process variation. Thus, the transition region (both stages on) can range from (V+) – 1.7 V to (V+) – 1.5 V on the low end, up to (V+) – 1.1 V to (V+) – 0.9 V on the high end. Within this transition region, PSRR, CMRR, offset voltage, offset drift, and THD may be degraded compared to device operation outside this region.

7.3.3 Input and ESD Protection

The OPAx314-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 33 shows how a series input resistor can be added to the driven input to limit the input current. The added resistor contributes thermal noise at the amplifier input and the value must be kept to a minimum in noise-sensitive applications.

Figure 33. Input Current Protection

7.3.4 Common-Mode Rejection Ratio (CMRR)

CMRR for the OPAx314-Q1 family is specified in several ways so the best match for a given application may be used; see the Electrical Characteristics table. First, the CMRR of the device in the common-mode range below the transition region [V_CM < (V+) – 1.3 V] is given. This specification is the best indicator of the capability of the device when the application requires use of one of the differential input pairs. Second, the CMRR over the entire common-mode range is specified at (V_CM = –0.2 V to 5.7 V). This last value includes the variations seen through the transition region, as shown in Figure 7.

7.3.5 EMI Susceptibility and Input Filtering

Op amps vary with regard to the susceptibility of the device to electromagnetic interference (EMI). If conducted EMI enters the op-amp, the dc offset observed at the amplifier output may shift from the nominal value while EMI is present. This shift is a result of signal rectification associated with the internal semiconductor junctions. While all op-amp pin functions can be affected by EMI, the signal input pins are likely to be the most susceptible. The OPAx314-Q1 family incorporates an internal input low-pass filter that reduces the amplifiers response to EMI. Both common-mode and differential mode filtering are provided by this filter. The filter is designed for a cutoff frequency of approximately 80 MHz (–3 dB), with a roll-off of 20 dB per decade.

Texas Instruments has developed the ability to accurately measure and quantify the immunity of an operational amplifier over a broad frequency spectrum extending from 10 MHz to 6 GHz. The EMI rejection ratio (EMIRR) metric allows op amps to be directly compared by the EMI immunity. Figure 32 shows the results of this testing on the OPAx314-Q1 family. Detailed information can also be found in the EMI Rejection Ratio of Operational Amplifiers application report, available for download from www.ti.com.

7.3.6 Rail-to-Rail Output

Designed as a micro-power, low-noise operational amplifier, the OPAx314-Q1 family 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. For resistive loads up to 10 kΩ, the output swings typically to within 5 mV of either supply rail regardless of the power-supply voltage applied. Different load conditions change the ability of the amplifier to swing close to the rails; see Figure 17.