OPA322-Q1_17 Datasheet, PDF (18/39 Pages) Texas Instruments – 20-MHz, Low-Noise, 1.8-V, RRI/O, CMOS Operational Amplifier

OPA322-Q1
OPA2322-Q1
OPA4322-Q1
SLOS856B – JUNE 2013 – REVISED MAY 2017
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Feature Description (continued)
For the circuit shown in Figure 28, the value of the variable feedback capacitor must be selected so that the input
resistance times the input capacitance of the OPAx322-Q1 (typically 9 pF) plus the estimated parasitic layout
capacitance equals the feedback capacitor times the feedback resistor with Equation 1.
R_IN × C_IN = R_F × C_F
where
• C_IN is equal to the OPAx322-Q1 input capacitance (sum of differential and common-mode) plus the layout
capacitance
(1)
The capacitor value can be adjusted until optimum performance is obtained.
8.3.5 EMI Susceptibility and Input Filtering
Operational amplifiers vary in susceptibility to electromagnetic interference (EMI). If conducted EMI enters the
device, 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 operational
amplifier pin functions can be affected by EMI, the input pins are likely to be the most susceptible. The OPAx322-
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 580 MHz (–3 dB), with a roll-off of 20 dB per decade.
8.3.6 Output Impedance
The open-loop output impedance of the OPAx322-Q1 common-source output stage is approximately 90 Ω. When
the op amp is connected with feedback, the loop gain significantly reduces this value. For each decade rise in
the closed-loop gain, the loop gain is reduced by the same amount, which results in a tenfold increase in
effective output impedance. While the OPAx322-Q1 output impedance remains flat over a wide frequency range.
At higher frequencies the output impedance rises as the open-loop gain of the op amp drops. However, at these
frequencies the output becomes capacitive as a result of parasitic capacitance. This characteristic prevents the
output impedance from becoming too high, which can cause stability problems when driving large capacitive
loads. As mentioned previously, the OPAx322-Q1 has excellent capacitive load drive capability for an op amp
with a bandwidth of this value.
8.3.7 Capacitive Load and Stability
The OPAx322-Q1 is designed to be used in applications where driving a capacitive load is required. As with all
op amps, there may be specific instances where the OPAx322-Q1 can become unstable. The particular op amp
circuit configuration, layout, gain, and output loading are some of the factors to consider when establishing
whether an amplifier is 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 become unstable than an amplifier operating 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 equivalent series resistance (ESR) of some very large capacitors (C_L > 1 µF) is sufficient to alter the phase
characteristics in the feedback loop so 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, as shown in Figure 29. One technique for
increasing the capacitive load drive capability of the amplifier operating in unity gain is to insert a small resistor
(R_S, typically 10-Ω to 20-Ω) in series with the output, as shown in Figure 30.
This resistor significantly reduces the overshoot and ringing associated with large capacitive loads. A possible
problem with this technique 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. However, the error contributed by the voltage divider may be insignificant. For
example, with a load resistance of R_L = 10 kΩ and R_S = 20 Ω, the gain error is approximately 0.2%. When R_L
decreases to 600 Ω (which the OPAx322-Q1 is able to drive), the error increases to 7.5%.
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