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
I
SC
+ = 40 mA and I
SC
- = –90 mA, typically, with ±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.
R
FB
FEEDBACK
R
X
332
A1
V
OUT
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 kΩ resistor in series with the noninverting input of the
OP285 cures the problem.
R
FB
*
V
IN
RS
3.92k
V
OUT
RL
2k
*R
FB
IS OPTIONAL
+
–
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 µs to recover to V
OUT
= +10 V
and approximately 1.5 µs to recover to V
OUT
= –10 V.
V
IN
4V p-p
@100 Hz
V
OUT
RL
2.43k
A1 = 1/2 OP285
R2
10k
R1
1k
1
2
3
A1
R
S
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.
