Datasheet

OPA320
V+
V
OUT
V
IN
R
IN
R C =R´ C´
IN IN F F
R
F
C
L
C
IN
C
IN
C
F
OPA320, OPA2320
OPA320S, OPA2320S
SBOS513E –AUGUST 2010–REVISED JUNE 2013 www.ti.com
FEEDBACK CAPACITOR IMPROVES result of signal rectification associated with the
RESPONSE internal semiconductor junctions. While all operational
amplifier pin functions can be affected by EMI, the
For optimum settling time and stability with high-
input pins are likely to be the most susceptible. The
impedance feedback networks, it may be necessary
OPA320 operational amplifier family incorporates an
to add a feedback capacitor across the feedback
internal input low-pass filter that reduces the
resistor, R
F
, as shown in Figure 35. This capacitor
amplifiers response to EMI. Both common-mode and
compensates for the zero created by the feedback
differential mode filtering are provided by the input
network impedance and the OPA320 input
filter. The filter is designed for a cut-off frequency of
capacitance (and any parasitic layout capacitance).
approximately 580MHz (–3dB), with a roll-off of 20dB
The effect becomes more significant with higher
per decade.
impedance networks.
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
NOTE: Where C
IN
is equal to the OPA320 input capacitance these frequencies the output also becomes capacitive
(approximately 9pF) plus any parasitic layout capacitance.
as a result of parasitic capacitance. This in turn
prevents the output impedance from becoming too
Figure 35. Feedback Capacitor Improves
high, which can cause stability problems when driving
Dynamic Performance
large capacitive loads. As mentioned previously, the
OPA320 has excellent capacitive load drive capability
It is suggested that a variable capacitor be used for
for an op amp with its bandwidth.
the feedback capacitor because input capacitance
may vary between op amps and layout capacitance is
CAPACITIVE LOAD AND STABILITY
difficult to determine. For the circuit shown in
The OPA320 is designed to be used in applications
Figure 35, the value of the variable feedback
where driving a capacitive load is required. As with all
capacitor should be chosen so that the input
op amps, there may be specific instances where the
resistance times the input capacitance of the OPA320
OPA320 can become unstable. The particular op amp
(typically 9pF) plus the estimated parasitic layout
circuit configuration, layout, gain, and output loading
capacitance equals the feedback capacitor times the
are some of the factors to consider when establishing
feedback resistor:
whether an amplifier is stable in operation. An op
R
IN
× C
IN
= R
F
× C
F
amp in the unity-gain (+1V/V) buffer configuration and
Where:
driving a capacitive load exhibits a greater tendency
to become unstable than an amplifier operated at a
C
IN
is equal to the OPA320 input capacitance
higher noise gain. The capacitive load, in conjunction
(sum of differential and common-mode) plus the
with the op amp output resistance, creates a pole
layout capacitance.
within the feedback loop that degrades the phase
The capacitor value can be adjusted until
margin. The degradation of the phase margin
optimum performance is obtained.
increases as the capacitive loading increases. When
operating in the unity-gain configuration, the OPA320
EMI SUSCEPTIBILITY AND INPUT FILTERING
remains stable with a pure capacitive load up to
Operational amplifiers vary in susceptibility to
approximately 1nF.
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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