Datasheet OP179, OP279 (Analog Devices) - 8

OP179/OP279 7 Table I. Snubber Networks for Large Capacitive Loads VS
ⴝ ⴞ
5V 6 RL
ⴝ
1k
⍀
Load Capacitance (CL) Snubber Network (RS, CS) TA
ⴝ
25
ⴗ
C
10 nF 20 Ω, 1 µF
5
100 nF 5 Ω, 10 µF
MHz – 4
1 µF 0 Ω, 10 µF
3 Overload Recovery Time
Overload, or overdrive, recovery time of an operational amplifier
BANDWIDTH 2
is the time required for the output voltage to recover to its linear region from a saturated condition. This recovery time is impor-
1
tant in applications where the amplifier must recover after a large transient event. The circuit in Figure 7 was used to
0 0.01 0.100 1 10
evaluate the OP179/OP279’s overload recovery time. The
CAPACITIVE LOAD – nF
OP179/OP279 takes approximately 1 µs to recover from positive Figure 4. OP179/OP279 Bandwidth vs. Capacitive Load saturation and approximately 1.2 µs to recover from negative saturation.
5V R2 R3 1k
⍀
10k
⍀
1/2 +5V OP279 VOUT V R IN S 100mV p-p 20V CL 1/2 C 10nF R1 V S OP279 OUT 909
⍀
1
␮
F RL 2V p-p 499
⍀
@ 100Hz –5V
Figure 5. Snubber Network Compensates for Capacitive Load The first step is to determine the value of the resistor, R Figure 7. Overload Recovery Time Test Circuit S. A good starting value is 100 Ω (typically, the optimum value will
Output Transient Current Recovery
be less than 100 Ω). This value is reduced until the small-signal In many applications, operational amplifiers are used to provide transient response is optimized. Next, CS is determined—10 µF moderate levels of output current to drive the inputs of ADCs, is a good starting point. This value is reduced to the smallest small motors, transmission lines and current sources. It is in these value for acceptable performance (typically, 1 µF). For the case applications that operational amplifiers must recover quickly to of a 10 nF load capacitor on the OP179/OP279, the optimal step changes in the load current while maintaining steady-state snubber network is a 20 Ω in series with 1 µF. The benefit is load current levels. Because of its high output current capability immediately apparent as seen in the scope photo in Figure 6. and low closed-loop output impedance, the OP179/OP279 is an The top trace was taken with a 10 nF load and the bottom trace excellent choice for these types of applications. For example, with the 20 Ω, 1 µF snubber network in place. The amount of when sourcing or sinking a 25 mA steady-state load current, the overshot and ringing is dramatically reduced. Table I illustrates a OP179/OP279 exhibits a recovery time of less than 500 ns to few sample snubber networks for large load capacitors. 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 10nF LOAD 100
In many data acquisition applications, the need for a precision
ONLY 90
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
SNUBBER
approach is that the largest single source of error in the circuit is
IN CIRCUIT 10 0%
the relative matching of the resistors used.
50mV 2
␮
s
The circuit illustrated in Figure 8 avoids the need for tightly matched resistors with the use of an active integrator circuit. In Figure 6. Overshoot and Ringing Are Reduced by Adding this circuit, the output of the voltage reference provides the a “Snubber” Network in Parallel with the 10 nF Load input drive for the integrator. The integrator, to maintain circuit equilibrium, adjusts its output to establish the proper relation- ship between the reference’s VOUT and GND. Thus, various negative output voltages can be chosen simply by substituting for the appropriate reference IC (see table). To speed up the –8– REV. G