OPA320 Datasheet, PDF (14/37 Pages) Texas Instruments – Precision, 20MHz, 0.9pA, Low-Noise, RRIO, CMOS Operational Amplifier with Shutdown

OPA320, OPA2320
OPA320S, OPA2320S
SBOS513E – AUGUST 2010 – 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 35. This capacitor
compensates for the zero created by the feedback
network impedance and the OPA320 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
OPA320
C_IN
V_OUT
C_L
NOTE: Where C_IN is equal to the OPA320 input capacitance
(approximately 9pF) plus any parasitic layout capacitance.
Figure 35. 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 35, the value of the variable feedback
capacitor should be chosen so that the input
resistance times the input capacitance of the OPA320
(typically 9pF) 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 OPA320 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 operational amplifier, the dc offset
observed at the amplifier output may shift from the
nominal value while EMI is present. This shift is a
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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
OPA320 operational amplifier family incorporates an
internal input low-pass filter that reduces the
amplifiers response to EMI. Both common-mode and
differential mode filtering are provided by the input
filter. The filter is designed for a cut-off frequency of
approximately 580MHz (–3dB), with a roll-off of 20dB
per decade.
OUTPUT IMPEDANCE
The open-loop output impedance of the OPA320
common-source output stage is approximately 90Ω.
When the op amp is connected with feedback, this
value is reduced significantly by the loop gain. For
example, with 130dB (typ) of open-loop gain, the
output impedance is reduced in unity-gain to less
than 0.03Ω. 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 OPA320 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 in turn
prevents the output impedance from becoming too
high, which can cause stability problems when driving
large capacitive loads. As mentioned previously, the
OPA320 has excellent capacitive load drive capability
for an op amp with its bandwidth.
CAPACITIVE LOAD AND STABILITY
The OPA320 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
OPA320 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 (+1V/V) buffer configuration and
driving a capacitive load exhibits a greater tendency
to become unstable than an amplifier operated 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 degradation of the phase margin
increases as the capacitive loading increases. When
operating in the unity-gain configuration, the OPA320
remains stable with a pure capacitive load up to
approximately 1nF.
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