REV. A

OP285

–7–

APPLICATIONS

Short-Circuit Protection

The OP285 has been designed with inherent short-circuit

protection to ground. An internal 30

Ω resistor, in series with

the output, limits the output current at room temperature to

±15 V supplies.

However, shorts to either supply may destroy the device when

excessive voltages or current are applied. If it is possible for a

user to short an output to a supply, for safe operation, the out-

put current of the OP285 should be design-limited to

±30 mA,

as shown in Figure 1.

FEEDBACK

332

A1

A1 = 1/2 OP285

–

+

Figure 1. Recommended Output Short-Circuit Protection

Input Over Current Protection

The maximum input differential voltage that can be applied

to the OP285 is determined by a pair of internal Zener diodes

connected across the inputs. They limit the maximum differ-

ential input voltage to

± 7.5 V. This is to prevent emitter-base

junction breakdown from occurring in the input stage of the

OP285 when very large differential voltages are applied. How-

ever, in order to preserve the OP285’s low input noise

voltage, internal resistance in series with the inputs were not

used to limit the current in the clamp diodes. In small-signal

applications, this is not an issue; however, in industrial appli-

cations, where large differential voltages can be inadvertently

applied to the device, large transient currents can be made to

flow through these diodes. The diodes have been designed to

carry a current of

± 8 mA; and, in applications where the

OP285’s differential voltage were to exceed

± 7.5 V, the resis-

tor values shown in Figure 2 safely limit the diode current to

± 8 mA.

A1

909

A1 = 1/2

909

–

+

Figure 2. OP285 Input Over Current Protection

Output Voltage Phase Reversal

Since the OP285’s input stage combines bipolar transistors

for low noise and p-channel JFETs for high speed performance,

the output voltage of the OP285 may exhibit phase reversal if

either of its inputs exceed its negative common-mode input

voltage. This might occur in very severe industrial applications

where a sensor or system fault might apply very large voltages on

the inputs of the OP285. Even though the input voltage range of

the OP285 is

±10.5 V, an input voltage of approximately –13.5 V

will cause output voltage phase reversal. In inverting amplifier

configurations, the OP285’s internal 7.5 V input clamping

diodes will prevent phase reversal; however, they will not prevent

this effect from occurring in noninverting applications. For these

applications, the fix is a simple one and is illustrated in Figure 3.

A 3.92 k

Ω resistor in series with the noninverting input of the

OP285 cures the problem.

RS

3.92k

RL

2k

+

–

Figure 3. Output Voltage Phase Reversal Fix

Overload or Overdrive Recovery

Overload or overdrive recovery time of an operational amplifier

is the time required for the output voltage to recover to a rated

output voltage from a saturated condition. This recovery time is

important in applications where the amplifier must recover quickly

after a large abnormal transient event. The circuit shown in Figure

4 was used to evaluate the OP285’s overload recovery time. The

OP285 takes approximately 1.2

and approximately 1.5

µs to recover to V

4V p-p

@100 Hz

RL

2.43k

A1 = 1/2 OP285

R2

10k

R1

1k

1

2

3

A1

909

Figure 4. Overload Recovery Time Test Circuit

Driving the Analog Input of an A/D Converter

Settling characteristics of operational amplifiers also include the

amplifier’s ability to recover, i.e., settle, from a transient output

current load condition. When driving the input of an A/D

converter, especially successive-approximation converters, the

amplifier must maintain a constant output voltage under

dynamically changing load current conditions. In these types of

converters, the comparison point is usually diode clamped, but

it may deviate several hundred millivolts resulting in high

frequency modulation of the A/D input current. Amplifiers that

exhibit high closed-loop output impedances and/or low unity-gain

crossover frequencies recover very slowly from output load

current transients. This slow recovery leads to linearity errors or

missing codes because of errors in the instantaneous input voltage.

Therefore, the amplifier chosen for this type of application should

exhibit low output impedance and high unity-gain bandwidth so

that its output has had a chance to settle to its nominal value

before the converter makes its comparison.

The circuit in Figure 5 illustrates a settling measurement circuit

for evaluating the recovery time of an amplifier from an output

load current transient. The amplifier is configured as a follower

with a very high speed current generator connected to its output.

In this test, a 1 mA transient current was used. As shown in

Figure 6, the OP285 exhibits an extremely fast recovery time of

139 ns to 0.01%. Because of its high gain-bandwidth product,

high open-loop gain, and low output impedance, the OP285 is

ideally suited to drive high speed A/D converters.