OPA2322A-Q1 Datasheet, PDF (13/23 Pages) Texas Instruments – 20-MHz, Low-Noise, 1.8-V, RRI/O, CMOS Operational Amplifier With Shutdown

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OPA322-Q1, OPA322S-Q1
OPA2322-Q1, OPA2322S-Q1
OPA4322-Q1, OPA4322S-Q1
SLOS856A – JUNE 2013 – REVISED JUNE 2013
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 32. This capacitor compensates for the
zero created by the feedback network impedance and the OPA322-Q1 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-Q1
C_IN
V_OUT
C_L
NOTE: Where C_IN is equal to the OPA322-Q1 input capacitance (approximately 9 pF) plus any parasitic layout capacitance.
Figure 32. Feedback Capacitor Improves Dynamic Performance
TI suggests the use of a variable capacitor for the feedback capacitor, because input capacitance may vary
between operational amplifiers and layout capacitance is difficult to determine. For the circuit shown in Figure 32,
choose the value of the variable feedback capacitor so that the input resistance times the input capacitance of
the OPA322-Q1 (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-Q1 input capacitance (sum of differential and common-mode) plus the layout
capacitance.
Adjust the capacitor value 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. Although EMI can
affect all operational amplifier pin functions, the input pins are likely to be the most susceptible. The OPA322-Q1
operational amplifier family incorporates an internal input low-pass filter that reduces the amplifier response to
EMI. The input filter proviede both common-mode and differential-mode filtering. The filter design is for a cutoff
frequency of approximately 580 MHz (–3 dB), with a rolloff of 20 dB per decade.
OUTPUT IMPEDANCE
The open-loop output impedance of the OPA322-Q1 common-source output stage is approximately 90 Ω.
Connecting the operational amplifier with feedback significantly reduces this value 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-Q1 output impedance remains very flat over a wide
frequency range, at higher frequencies the output impedance rises as the open-loop gain of the operational
amplifier 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-Q1 has
excellent capacitive load-drive capability for an operational amplifier with its bandwidth.
Copyright © 2013, Texas Instruments Incorporated
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Product Folder Links: OPA322-Q1 OPA322S-Q1 OPA2322-Q1 OPA2322S-Q1 OPA4322-Q1 OPA4322S-Q1