OPA314_15 Datasheet, PDF (18/45 Pages) Texas Instruments – OPAx314 3-MHz, Low-Power, Low-Noise, RRIO, 1.8-V CMOS Operational Amplifier

OPA314, OPA2314, OPA4314
SBOS563G – MAY 2011 – REVISED JUNE 2015
www.ti.com
Feature Description (continued)
7.3.4 Common-Mode Rejection Ratio (CMRR)
CMRR for the OPA314 is specified in several ways so the best match for a given application may be used; see
the Electrical Characteristics. 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 (see Figure 7).
7.3.5 EMI Susceptibility and Input Filtering
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 may shift from its
nominal value while EMI is present. This shift is a result of signal rectification associated with the internal
semiconductor junctions. While all operational amplifier pin functions can be affected by EMI, the signal input
pins are likely to be the most susceptible. The OPA314 operational amplifier family incorporate 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 operational amplifiers to be directly compared by the EMI immunity. Figure 32 illustrates the results
of this testing on the OPAx314. Detailed information can also be found in the application report, EMI Rejection
Ratio of Operational Amplifiers (SBOA128), available for download from www.ti.com.
7.3.6 Rail-to-Rail Output
Designed as a micro-power, low-noise operational amplifier, the OPA314 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; refer to Figure 17.
7.3.7 Capacitive Load and Stability
The OPA314 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 OPA314 can become unstable. The particular
operational amplifiers 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. When operating in the unity-gain configuration, the
OPA314 remains stable with a pure capacitive load up to approximately 1 nF. The equivalent series resistance
(ESR) of some very large capacitors (C_L 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 Figure 20.
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 35.
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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