Datasheet OP285 (Analog Devices) - 7

OP285 APPLICATIONS
applications, the fix is a simple one and is illustrated in Figure 3.
Short-Circuit Protection
A 3.92 kΩ resistor in series with the noninverting input of the The OP285 has been designed with inherent short-circuit OP285 cures the problem. protection to ground. An internal 30 Ω resistor, in series with the output, limits the output current at room temperature to
RFB*
ISC+ = 40 mA and ISC- = –90 mA, typically, with ± 15 V supplies. However, shorts to either supply may destroy the device when
– VOUT
excessive voltages or current are applied. If it is possible for a
VIN +
user to short an output to a supply, for safe operation, the out-
RS RL 3.92k
put current of the OP285 should be design-limited to ± 30 mA,
2k
as shown in Figure 1.
*RFB IS OPTIONAL RFB
Figure 3. Output Voltage Phase Reversal Fix
FEEDBACK Overload or Overdrive Recovery RX –
Overload or overdrive recovery time of an operational amplifier
332 A1
is the time required for the output voltage to recover to a rated
VOUT +
output voltage from a saturated condition. This recovery time is
A1 = 1/2 OP285
important in applications where the amplifier must recover quickly Figure 1. Recommended Output Short-Circuit Protection after a large abnormal transient event. The circuit shown in Figure 4 was used to evaluate the OP285’s overload recovery time. The
Input Over Current Protection
OP285 takes approximately 1.2 µs to recover to VOUT = +10 V The maximum input differential voltage that can be applied and approximately 1.5 µs to recover to V to the OP285 is determined by a pair of internal Zener diodes OUT = –10 V. connected across the inputs. They limit the maximum differ-
R1 R2
ential input voltage to ± 7.5 V. This is to prevent emitter-base
1k 10k
junction breakdown from occurring in the input stage of the
2
OP285 when very large differential voltages are applied. How-
VOUT 1 A1
ever, in order to preserve the OP285’s low input noise
3
voltage, internal resistance in series with the inputs were not
V RS IN RL
used to limit the current in the clamp diodes. In small-signal
4V p-p 909 2.43k @100 Hz
applications, this is not an issue; however, in industrial appli-
A1 = 1/2 OP285
cations, where large differential voltages can be inadvertently applied to the device, large transient currents can be made to Figure 4. Overload Recovery Time Test Circuit flow through these diodes. The diodes have been designed to carry a current of ± 8 mA; and, in applications where the
Driving the Analog Input of an A/D Converter
OP285’s differential voltage were to exceed ± 7.5 V, the resis- Settling characteristics of operational amplifiers also include the tor values shown in Figure 2 safely limit the diode current to amplifier’s ability to recover, i.e., settle, from a transient output ± 8 mA. 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
909
dynamically changing load current conditions. In these types of
–
converters, the comparison point is usually diode clamped, but
A1
it may deviate several hundred millivolts resulting in high
909 +
frequency modulation of the A/D input current. Amplifiers that exhibit high closed-loop output impedances and/or low unity-gain
A1 = 1/2
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. Figure 2. OP285 Input Over Current Protection Therefore, the amplifier chosen for this type of application should
Output Voltage Phase Reversal
exhibit low output impedance and high unity-gain bandwidth so Since the OP285’s input stage combines bipolar transistors that its output has had a chance to settle to its nominal value for low noise and p-channel JFETs for high speed performance, before the converter makes its comparison. the output voltage of the OP285 may exhibit phase reversal if The circuit in Figure 5 illustrates a settling measurement circuit either of its inputs exceed its negative common-mode input for evaluating the recovery time of an amplifier from an output voltage. This might occur in very severe industrial applications load current transient. The amplifier is configured as a follower where a sensor or system fault might apply very large voltages on with a very high speed current generator connected to its output. the inputs of the OP285. Even though the input voltage range of In this test, a 1 mA transient current was used. As shown in the OP285 is ± 10.5 V, an input voltage of approximately –13.5 V Figure 6, the OP285 exhibits an extremely fast recovery time of will cause output voltage phase reversal. In inverting amplifier 139 ns to 0.01%. Because of its high gain-bandwidth product, configurations, the OP285’s internal 7.5 V input clamping high open-loop gain, and low output impedance, the OP285 is diodes will prevent phase reversal; however, they will not prevent ideally suited to drive high speed A/D converters. this effect from occurring in noninverting applications. For these REV. C –7–