OPA322_1111 Datasheet, PDF (13/34 Pages) Texas Instruments – 20-MHz, Low-Noise, 1.8-V, RRI/O, CMOS Operational Amplifier with Shutdown

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OPA322, OPA322S
OPA2322, OPA2322S
OPA4322, OPA4322S
SBOS538C – JANUARY 2011 – REVISED NOVEMBER 2011
FEEDBACK CAPACITOR IMPROVES RESPONSE
For optimum settling time and stability with high-impedance feedback networks, it may be necessary to add a
feedback capacitor across the feedback resistor, R_F, as shown in Figure 28. This capacitor compensates for the
zero created by the feedback network impedance and the OPA322 input capacitance (and any parasitic layout
capacitance). The effect becomes more significant with higher impedance networks.
C_F
R_IN
R_F
V_IN
V+
R_IN ´ C_IN = R_F ´ C_F
C_IN
OPA322
C_IN
V_OUT
C_L
NOTE: Where C_IN is equal to the OPA322 input capacitance (approximately 9 pF) plus any parasitic layout capacitance.
Figure 28. Feedback Capacitor Improves Dynamic Performance
It is suggested that a variable capacitor be used for the feedback capacitor because input capacitance may vary
between op amps and layout capacitance is difficult to determine. For the circuit shown in Figure 28, the value of
the variable feedback capacitor should be chosen so that the input resistance times the input capacitance of the
OPA322 (typically 9 pF) plus the estimated parasitic layout capacitance equals the feedback capacitor times the
feedback resistor:
R_IN × C_IN = R_F × C_F
Where:
C_IN is equal to the OPA322 input capacitance (sum of differential and common-mode) plus the layout
capacitance.
The capacitor value can be adjusted until optimum performance is obtained.
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 OPA322
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.
OUTPUT IMPEDANCE
The open-loop output impedance of the OPA322 common-source output stage is approximately 90 Ω. When the
op amp is connected with feedback, this value is reduced significantly by the loop gain. For each decade rise in
the closed-loop gain, the loop gain is reduced by the same amount, which results in a ten-fold increase in
effective output impedance. While the OPA322 output impedance remains very 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 also becomes capacitive as a result of parasitic capacitance. This characteristic, in turn,
prevents the output impedance from becoming too high, which can cause stability problems when driving large
capacitive loads. As mentioned previously, the OPA322 has excellent capacitive load drive capability for an op
amp with its bandwidth.
Copyright © 2011, Texas Instruments Incorporated
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