OP184_06 Datasheet, PDF (15/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

OP184/OP284/OP484
5
4
3
2
1
0
–1
–2
–3
–4
–5
–5 –4 –3 –2 –1 0 1 2 3 4 5
INPUT VOLTAGE (V)
Figure 46. Input Overvoltage I-V Characteristics of the OP284
As shown in Figure 46, internal p-n junctions to the OP284
energize 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 44, 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 current
quickly exceeds 5 mA. If this condition continues to exist, an
external series resistor should be added at the expense of
additional thermal noise. Figure 47 illustrates a typical
noninverting configuration for an overvoltage-protected
amplifier where the series resistance, RS, is chosen such that
R_S
=
V_{IN (MAX )} − V_SUPPLY
5 mA
For example, a 1 kΩ resistor protects 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,
to ensure optimum dc and ac performance, it is recommended
to balance source impedance levels.
R2
1/2
R1
OP284
V_IN
V_OUT
Figure 47. Resistance in Series with Input Limits Overvoltage Currents
to Safe Values
OUTPUT PHASE REVERSAL
Some operational amplifiers designed for single-supply
operation 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
determines 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 excursions from exceeding the device’s negative
supply (that is, GND), preventing a condition that causes the
output voltage to change phase. JFET-input amplifiers can also
exhibit phase reversal, and, if so, a series input resistor is usually
required to prevent it.
The OP284 is free from reasonable input voltage range
restrictions, provided that input voltages no greater than the
supply voltages are applied. Although device output does not
change phase, large currents can flow through the input
protection diodes as shown in Figure 46. Therefore, the technique
recommended in the Input Overvoltage Protection section
should be applied to 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 OPx84 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.
Referring to the op amp noise model circuit configuration
illustrated in Figure 48, the expression for an amplifier’s total
equivalent input noise voltage for a source resistance level, RS,
is given by
[ ] e_nT = 2 (e_nR )2 + (i_nOA × R)² + (e_nOA )² , units in
V
Hz
where:
RS = 2R is the effective, or equivalent, circuit source resistance.
(enOA)² is the op amp equivalent input noise voltage spectral
power (1 Hz BW).
(inOA)² is the op amp equivalent input noise current spectral
power (1 Hz BW).
(enR)² is the source resistance thermal noise voltage power (4 kTR).
k = Boltzmann’s constant = 1.38 × 10^–23 J/K.
T is the ambient temperature in Kelvins of the circuit = 273.15 +
TA (°C).
Rev. D | Page 15 of 24