OP184 Datasheet, PDF (11/20 Pages) Analog Devices – Precision Rail-to-Rail Input & Output Operational Amplifiers

OP184/OP284/OP484
Thus, the saturation voltage of the output transistors sets the
limit on the OP284’s maximum output voltage swing. Output
short circuit current limiting is determined by the maximum
signal current into the base of Q1 from the second gain stage.
Under output short circuit conditions, this input current level is
approximately 100 µA. With transistor current gains around
200, the short circuit current limits are typically 20 mA. The
output stage also exhibits voltage gain. This is accomplished by
use of common-emitter amplifiers, and as a result the voltage
gain of the output stage (thus, the open-loop gain of the device)
exhibits a dependence to the total load resistance at the output
of the OP284.
Input Overvoltage Protection
As with any semiconductor device, if conditions exist where the
applied input voltages to the device exceed either supply voltage,
then the device’s input overvoltage I-V characteristic must be
considered. When an overvoltage occurs, the amplifier could be
damaged depending on the magnitude of the applied voltage
and the magnitude of the fault current. Figure 43 illustrates the
over voltage I-V characteristic of the OP284. This graph was
generated with the supply pins connected to GND and a curve
tracer’s collector output drive connected to the input.
5
4
3
2
1
0
–1
–2
–3
–4
–5
–5 –4 –3 –2 –1 0 1 2 3 4 5
INPUT VOLTAGE – Volts
Figure 43. Input Overvoltage I-V Characteristics of the
OP284
As shown in the figure, internal p-n junctions to the OP284 en-
ergize and permit current flow from the inputs to the supplies
when the input is 1.8 V more positive and 0.6 V more negative
than the respective supply rails. As illustrated in the simplified
equivalent circuit shown in Figure 41, the OP284 does not have
any internal current limiting resistors; thus, fault currents can
quickly rise to damaging levels.
This input current is not inherently damaging to the device,
provided that it is limited to 5 mA or less. For the OP284, once
the input exceeds the negative supply by 0.6 V, the input cur-
rent quickly exceeds 5 mA. If this condition continues to exist,
an external series resistor should be added at the expense of ad-
ditional thermal noise. Figure 44 illustrates a typical noninvert-
ing configuration for an overvoltage protected amplifier where
the series resistance, R_S, is chosen such that:
R_S
=
V_IN
( MAX ) ^{– V}SUPPLY
5 mA
R2
1/2
R1
OP284
V_IN
V_OUT
Figure 44. A Resistance in Series with an Input Limits
Overvoltage Currents to Safe Values
For example, a 1 kΩ resistor will protect the OP284 against
input signals up to 5 V above and below the supplies. For other
configurations where both inputs are used, then each input
should be protected against abuse with a series resistor. Again,
in order to ensure optimum dc and ac performance, it is recom-
mended to balance source impedance levels. For more informa-
tion on the general overvoltage characteristics of amplifiers,
please refer to the 1993 System Applications Guide, Section 1,
pages 56-69. This reference textbook is available from the Ana-
log Devices Literature Center.
Output Phase Reversal
Some operational amplifiers designed for single-supply opera-
tion exhibit an output voltage phase reversal when their inputs
are driven beyond their useful common-mode range. Typically
for single-supply bipolar op amps, the negative supply deter-
mines the lower limit of their common-mode range. With these
devices, external clamping diodes, with the anode connected to
ground and the cathode to the inputs, prevent input signal ex-
cursions from exceeding the device’s negative supply (i.e.,
GND), preventing a condition that could cause the output volt-
age to change phase. JFET-input amplifiers may also exhibit
phase reversal, and, if so, a series input resistor is usually re-
quired to prevent it.
The OP284 is free from reasonable input voltage range restric-
tions provided that the input voltages no greater than the supply
voltages are applied. Although the device’s output will not
change phase, large currents can flow through the input protec-
tion diodes, as was shown in Figure 43. Therefore, the technique
recommended in the Input Overvoltage Protection section
should be applied in those applications where the likelihood of
input voltages exceeding the supply voltages is high.
Designing Low Noise Circuits in Single Supply Applications
In single supply applications, devices like the OP284 extend the
dynamic range of the application through the use of rail-to-rail
operation. In fact, the OP284 family is the first of its kind to
combine single supply, rail-to-rail operation and low noise in
one device. It is the first device in the industry to exhibit an
input noise voltage spectral density of less than 4 nV/√Hz at
1 kHz. It was also designed specifically for low-noise, single-
supply applications, and as such some discussion on circuit
noise concepts in single supply applications is appropriate.
REV. 0
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