OPA2180
OPA4180
SBOS584B –NOVEMBER 2011–REVISED DECEMBER 2011
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GENERAL LAYOUT GUIDELINES
For best operational performance of the device, good printed circuit board (PCB) layout practices are
recommended. Low-loss, 0.1-µF bypass capacitors should be connected between each supply pin and ground,
placed as close to the device as possible. A single bypass capacitor from V+ to ground is applicable to
single-supply applications.
PHASE-REVERSAL PROTECTION
The OPAx180 family has an internal phase-reversal protection. Many op amps exhibit a phase reversal when the
input is driven beyond its linear common-mode range. This condition is most often encountered in noninverting
circuits when the input is driven beyond the specified common-mode voltage range, causing the output to
reverse into the opposite rail. The input of the OPAx180 prevents phase reversal with excessive common-mode
voltage. Instead, the output limits into the appropriate rail. This performance is shown in Figure 26.
+18 V
Device
-18 V
37 VPP
Sine Wave
(±18.5 V)
VIN
VOUT
Time (100 ms/div)
Figure 26. No Phase Reversal
CAPACITIVE LOAD AND STABILITY
The dynamic characteristics of the OPAx180 have been optimized for a range of common operating conditions.
The combination of low closed-loop gain and high capacitive loads decreases the phase margin of the amplifier
and can lead to gain peaking or oscillations. As a result, heavier capacitive loads must be isolated from the
output. The simplest way to achieve this isolation is to add a small resistor (for example, ROUT equal to 50 Ω) in
series with the output. Figure 27 and Figure 28 illustrate graphs of small-signal overshoot versus capacitive load
for several values of ROUT. Also, refer to the Applications Report, Feedback Plots Define Op Amp AC
Performance (SBOA015), available for download from the TI website, for details of analysis techniques and
application circuits.
40
35
30
25
20
15
10
5
40
35
30
25
20
15
10
5
RL = 10 kW
ROUT = 0 W
ROUT = 25 W
ROUT = 50 W
ROUT = 0 W
ROUT = 25 W
ROUT = 50 W
G = +1
+18
V
RF = 10 kW
RI = 10 kW
G = -1
ROUT
+18 V
Device
ROUT
RL
CL
-18
V
Device
CL
RL = 10 kW
-18 V
0
0
0
100 200 300 400 500 600 700 800 900 1000
Capacitive Load (pF)
0
100 200 300 400 500 600 700 800 900 1000
Capacitive Load (pF)
Figure 27. Small-Signal Overshoot versus
Capacitive Load (100-mV Output Step)
Figure 28. Small-Signal Overshoot versus
Capacitive Load (100-mV Output Step)
12
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