OP484_15 Datasheet, PDF (15/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

5
4
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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 addi-
tional 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 configu-
rations where both inputs are used, each input should be protected
against abuse with a series resistor. Again, to ensure optimum
dc and ac performance, it is recommended that source impedance
levels be balanced.
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,
OP184/OP284/OP484
external clamping diodes, with the anode connected to ground
and the cathode to the inputs, prevent input signal excursions
from exceeding the negative supply of the device (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 appli-
cations 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 S} )² + (e_nOA )² , units in
V
Hz
where:
RS = 2R is the effective, or equivalent, circuit source resistance.
(enR)² is the source resistance thermal noise voltage power (4kTR).
k is the Boltzmann’s constant = 1.38 × 10^–23 J/K.
T is the ambient temperature in Kelvins of the circuit = 273.15 +
TA (°C).
(inOA)² is the op amp equivalent input noise current spectral power
(1 Hz bandwidth).
(enOA)² is the op amp equivalent input noise voltage spectral power
(1 Hz bandwidth).
R
e_nR
e_nOA
NOISELESS
i_nOA
R
e_nR
NOISELESS
i_nOA
IDEAL
NOISELESS
OP AMP
R_S = 2R
Figure 48. Op Amp Noise Circuit Model Used to Determine Total Circuit
Equivalent Input Noise Voltage and Noise Figure
Rev. J | Page 15 of 24