OPA320 Datasheet, PDF (14/31 Pages) Texas Instruments – Precision, 20MHz, 0.9pA, Low-Noise, RRIO, CMOS Operational Amplifier with Shutdown

OPA320, OPA2320
OPA320S, OPA2320S
SBOS513D – AUGUST 2010 – REVISED NOVEMBER 2011
The equivalent series resistance (ESR) of some very
large capacitors (C_L > 1µF) is sufficient to alter the
phase characteristics in the feedback loop such that
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 33. 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 10Ω to 20Ω, in series with the output,
as shown in Figure 32.
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. The error
contributed by the voltage divider may be
insignificant. For instance, with a load resistance, R_L
= 10kΩ and R_S = 20Ω, the gain error is only about
0.2%. However, when R_L is decreased to 600Ω,
which the OPA320 is able to drive, the error
increases to 7.5%.
V+
R_S
OPA320
V_OUT
V_IN
10W to
20W
R_L
C_L
Figure 32. Improving Capacitive Load Drive
70
G = 1, V_S = 1.8V
60
G = 1, V_S = 5.5V
50
G = 10, V_S = 1.8V
G = 10, V_S = 5.5V
40
30
20
10
0
0
500 1000 1500 2000 2500 3000
Capacitive Load (pF)
Figure 33. Small-Signal Overshoot versus
Capacitive Load (100mV_PPoutput step)
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OVERLOAD RECOVERY TIME
Overload recovery time is the time it takes the output
of the amplifier to come out of saturation and recover
to the linear region. Overload recovery is particularly
important in applications where small signals must be
amplified in the presence of large transients.
Figure 34 and Figure 35 show the positive and
negative overload recovery times of the OPA320,
respectively. In both cases, the time elapsed before
the OPA320 comes out of saturation is less than
100ns. In addition, the symmetry between the positive
and negative recovery times allows excellent signal
rectification without distortion of the output signal.
3
2.5
2
1.5
1
0.5
0
-0.5
-1
9.75
Output
V_S = ±2.75V
G = -10
Input
10
10.25
10.5
10.75
11
Time (250ns/div)
Figure 34. Positive Recovery Time
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
-3
9.75
Input
Output
V_S = ±2.75V
G = -10
10
10.25
10.5
10.75
11
Time (250ns/div)
Figure 35. Negative Recovery Time
14
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