The OPA316 is a family of low-power, rail-to-rail input and output operational amplifiers. These devices operate from 1.8 V to 5.5 V, are unity-gain stable, and are suitable for a wide range of general-purpose applications. The class AB output stage is capable of driving less than or equal to 10-kΩ loads connected to any point between V+ and ground. The input common-mode voltage range includes both rails and allows the OPA316 series to be used in virtually any single-supply application. Rail-to-rail input and output swing significantly increases dynamic range, especially in low-supply applications, and makes them ideal for driving sampling analog-to-digital converters (ADCs).
The OPA316 family features 10-MHz bandwidth and 6-V/μs slew rate with only 400-μA supply current per channel, providing good ac performance at very-low power consumption. DC applications are well served with a very-low input noise voltage of 11 nV/√Hz at 1 kHz, low input bias current (5 pA), and an input offset voltage of 0.5 mV (typical).
The OPAx316 operational amplifiers are 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 illustrated in the Typical Characteristics graphs.
The input common-mode voltage range of the OPAx316 series extends 200 mV beyond the supply rails for supply voltages greater than 2.5 V. This performance is achieved with a complementary input stage: an N-channel input differential pair in parallel with a P-channel differential pair, as shown in the Functional Block Diagram. The N-channel pair is active for input voltages close to the positive rail, typically (V+) – 1.4 V to 200 mV above the positive supply, whereas the P-channel pair is active for inputs from 200 mV below the negative supply to approximately (V+) – 1.4 V. There is a small transition region, typically (V+) – 1.2 V to (V+) – 1 V, in which both pairs are on. This 200-mV transition region can vary up to 200 mV with process variation. Thus, the transition region (both stages on) can range from (V+) – 1.4 V to (V+) – 1.2 V on the low end, up to (V+) – 1 V to (V+) – 0.8 V on the high end. Within this transition region, PSRR, CMRR, offset voltage, offset drift, and THD can be degraded compared to device operation outside this region.
The OPAx316 incorporates internal 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 provide in-circuit, input overdrive protection, as long as the current is limited to 10 mA as stated in Absolute Maximum Ratings. Figure 37 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.
CMRR for the OPAx316 is specified in several ways so the user can select the best match for a given application, as shown in Electrical Characteristics. First, the data sheet gives the CMRR of the device in the common-mode range below the transition region [VCM < (V+) – 1.4 V]. This specification is the best indicator of device capability when the application requires use of one of the differential input pairs. Second, the CMRR over the entire common-mode range is specified at VCM = –0.2 V to 5.7 V for VS = 5.5 V. This last value includes the variations shown in Figure 4 through the transition region.
Operational amplifiers vary with regard to the susceptibility of the device to electromagnetic interference (EMI). If conducted EMI enters the operational amplifier, the dc offset observed at the amplifier output can shift from its nominal value when EMI is present. This shift is a result of signal rectification associated with the internal semiconductor junctions. Although all operational amplifier pin functions can be affected by EMI, the signal input pins are likely to be the most susceptible. The OPA316 operational amplifier family incorporates an internal input low-pass filter that reduces the amplifier response to EMI. This filter provides both common-mode and differential-mode filtering. The filter is designed for a cutoff frequency of approximately 80 MHz (–3 dB), with a roll-off of 20 dB per decade.
TI 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 operational amplifiers to be directly compared by the EMI immunity. Figure 35 illustrates the results of this testing on the OPA316 series. For more information, see EMI Rejection Ratio of Operational Amplifiers (SBOA128).
Designed as a low-power, low-noise operational amplifier, the OPAx316 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 of 10-kΩ, the output swings typically to within 30 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 the typical characteristic graph Output Voltage Swing vs Output Current (Figure 11).
The OPAx316 is designed to be used in applications where driving a capacitive load is required. As with all operational amplifiers, there may be specific instances where the OPAx316 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 as the capacitive loading increases. As a conservative best practice, designing for 25% overshoot (40° phase margin) provides improved stability over process variations. The equivalent series resistance (ESR) of some very-large capacitors (CL greater than 1 μF) 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 observing the overshoot response of the amplifier at higher voltage gains. See the typical characteristic graphs, Small-Signal Overshoot vs Capacitive Load (Figure 24, G = –1 V/V) and Small-Signal Overshoot vs Capacitive Load (Figure 25, G = +1 V/V).
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 38. 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.
Overload recovery is defined as the time required for the operational amplifier output to recover from a saturated state to a linear state. The output devices of the operational amplifier enter a saturation region when the output voltage exceeds the rated operating voltage, either because of the high input voltage or the high gain. After the device enters the saturation region, the charge carriers in the output devices require time to return back to the linear state. After the charge carriers return back to the linear state, the device begins to slew at the specified slew rate. Thus, the propagation delay in case of an overload condition is the sum of the overload recovery time and the slew time. The overload recovery time for the OPAx316 is approximately 300 ns.
The OPA2316 (dual version) device uses the DFN style package (also known as SON); this package is a QFN with contacts on only two sides of the package bottom. This leadless package maximizes printed circuit board (PCB) space and offers enhanced thermal and electrical characteristics through an exposed pad. One of the primary advantages of the DFN package is the low, 0.9-mm height. DFN packages are physically small, have a smaller routing area, improved thermal performance, reduced electrical parasitics, and use a pinout scheme that is consistent with other commonly-used packages, such as SOIC and MSOP). Additionally, the absence of external leads eliminates bent-lead issues.
Connect the exposed lead frame die pad on the bottom of the DFN package to the most negative potential (V–).
The OPA316, OPA2316, and OPA4316 devices are powered on when the supply is connected. The devices can be operated as a single-supply operational amplifier or a dual-supply amplifier, depending on the application.
The OPA2316S device has a SHDN (enable) pin function referenced to the negative supply voltage of the operational amplifier. A logic level high enables the operational amplifier. A valid logic high is defined as voltage [(V+) – 0.1 V], up to (V+), applied to the SHDN pin. A valid logic low is defined as [(V–) + 0.1 V], down to (V–), applied to the enable pin. The maximum allowed voltage applied to SHDN is 5.5 V with respect to the negative supply, independent of the positive supply voltage. Connect this pin to a valid high or a low voltage or driven, but not left as an open circuit.
The logic input is a high-impedance CMOS input. Both inputs are independently controlled. For battery-operated applications, this feature can be used to greatly reduce the average current and extend battery life.