OP179 Datasheet, PDF (8/16 Pages) Analog Devices – Rail-to-Rail High Output Current Operational Amplifiers

OP179/OP279
7
V_S = ؎5V
6
R_L = 1k⍀
T_A = +25؇C
5
4
3
2
1
0
0.01
0.100
1
10
CAPACITIVE LOAD – nF
Figure 25. OP179/OP279 Bandwidth vs. Capacitive Load
+5V
V_IN
100mV p-p
1/2
OP279
R_S
20V
C_S
1␮F
V_OUT
C_L
10nF
Figure 26. Snubber Network Compensates for Capacitive
Load
The first step is to determine the value of the resistor, R_S. A
good starting value is 100 Ω (typically, the optimum value will
be less than 100 Ω). This value is reduced until the small-signal
transient response is optimized. Next, C_S is determined—10 µF
is a good starting point. This value is reduced to the smallest
value for acceptable performance (typically, 1 µF). For the case
of a 10 nF load capacitor on the OP179/OP279, the optimal
snubber network is a 20 Ω in series with 1 µF. The benefit is
immediately apparent as seen in the scope photo in Figure 27.
The top trace was taken with a 10 nF load and the bottom trace
with the 20 Ω, 1 µF snubber network in place. The amount of
overshot and ringing is dramatically reduced. Table I illustrates
a few sample snubber networks for large load capacitors.
10nF LOAD ₁₀₀
ONLY ₉₀
SNUBBER
IN CIRCUIT ¹⁰
0%
50mV
2␮s
Figure 27. Overshoot and Ringing Is Reduced by Adding a
“Snubber” Network in Parallel with the 10 nF Load
Table I. Snubber Networks for Large Capacitive Loads
Load Capacitance (C_L)
10 nF
100 nF
1 µF
Snubber Network (R_S, C_S)
20 Ω, 1 µF
5 Ω, 10 µF
0 Ω, 10 µF
Overload Recovery Time
Overload, or overdrive, recovery time of an operational amplifier
is the time required for the output voltage to recover to its linear
region from a saturated condition. This recovery time is impor-
tant in applications where the amplifier must recover after a
large transient event. The circuit in Figure 28 was used to
evaluate the OP179/OP279’s overload recovery time. The
OP179/OP279 takes approximately 1 µs to recover from positive
saturation and approximately 1.2 µs to recover from negative
saturation.
R2
1k⍀
R3
10k⍀
+5V
2V p-p
@ 100Hz
R1
909⍀
1/2
OP279
–5V
R_L
499⍀
V_OUT
Figure 28. Overload Recovery Time Test Circuit
Output Transient Current Recovery
In many applications, operational amplifiers are used to provide
moderate levels of output current to drive the inputs of ADCs,
small motors, transmission lines and current sources. It is in
these applications that operational amplifiers must recover
quickly to step changes in the load current while maintaining
steady-state load current levels. Because of its high output
current capability and low closed-loop output impedance, the
OP179/OP279 is an excellent choice for these types of applica-
tions. For example, when sourcing or sinking a 25 mA steady-
state load current, the OP179/OP279 exhibits a recovery time of
less than 500 ns to 0.1% for a 10 mA (i.e., 25 mA to 35 mA and
35 mA to 25 mA) step change in load current.
A Precision Negative Voltage Reference
In many data acquisition applications, the need for a precision
negative reference is required. In general, any positive voltage
reference can be converted into a negative voltage reference
through the use of an operational amplifier and a pair of matched
resistors in an inverting configuration. The disadvantage to that
approach is that the largest single source of error in the circuit is
the relative matching of the resistors used.
The circuit illustrated in Figure 29 avoids the need for tightly
matched resistors with the use of an active integrator circuit. In
this circuit, the output of the voltage reference provides the
input drive for the integrator. The integrator, to maintain cir-
cuit equilibrium, adjusts its output to establish the proper rela-
tionship between the reference’s V_OUT and GND. Thus, various
negative output voltages can be chosen simply by substituting
for the appropriate reference IC (see table). To speed up the
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REV. F