OP179/OP279

–8–

REV. F

7

2

0

0.01

0.100

10

1

5

1

3

4

6

CAPACITIVE LOAD – nF

Figure 25. OP179/OP279 Bandwidth vs. Capacitive Load

1/2

OP279

20V

1 F

10nF

+5V

100mV p-p

Figure 26. Snubber Network Compensates for Capacitive

Load

good starting value is 100

Ω (typically, the optimum value will

be less than 100

Ω). This value is reduced until the small-signal

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.

90

100

10nF LOAD

ONLY

SNUBBER

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

10 nF

20

Ω, 1 µF

100 nF

5

Ω, 10 µF

1

µF0 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.

1/2

OP279

499

+5V

–5V

R3

10k

R2

1k

R1

909

2V p-p

@ 100Hz

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-

negative output voltages can be chosen simply by substituting

for the appropriate reference IC (see table). To speed up the