ADA4084-2ARMZ AD [Analog Devices], ADA4084-2ARMZ Datasheet - Page 21

• Manufacturer Part Number: ADA4084-2ARMZ
• Description: 30 V, Low Noise, Rail-to-Rail
• Manufacturer: AD [Analog Devices]

Data Sheet
INPUT PROTECTION
As with any semiconductor device, if conditions exist where the
applied input voltages to the device exceed either supply voltage,
the input overvoltage I-to-V characteristic of the device must be
considered. When an overvoltage occurs, the amplifier may be
damaged, depending on the magnitude of the applied voltage
and the magnitude of the fault current.
The D1, D2, D4, and D5 diodes conduct when the input common-
mode voltage exceeds either supply pin by a diode drop. This
varies with temperature and is in the range of 0.3 V to 0.8 V. As
illustrated in the simplified equivalent circuit shown in Figure 73,
the
tors; 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. If a fault condition
causes more than 5 mA to flow, an external series resistor
should be added at the expense of additional thermal noise.
Figure 75 illustrates a typical noninverting configuration for an
overvoltage-protected amplifier where the series resistance, R
is chosen, such that
For example, a 1 kΩ resistor protects the
input signals up to 5 V above and below the supplies. Note that
the thermal noise of a 1 kΩ resistor at room temperature is
4 nV/√Hz, which exceeds the voltage noise of the ADA4084-2.
For other configurations 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.
V
To protect Q1-Q2 and Q3-Q4 from large differential voltages
that may result in Zener breakdown of the emitter-base junction,
D100 and D101 are connected between the two inputs. This
precludes operation as a comparator. For a more complete
description, see the
Single-Supply, and Rail-to-Rail Issues; the
Comparators, the
Comparators; and the
Amps as Comparators, at www.analog.com.
IN
ADA4084-2
R
S
=
R1
V
IN
(
Limits Overvoltage Currents to Safe Values
MAX
Figure 75. Resistance in Series with Input
ADA4084-2
does not have any internal current limiting resis-
5
MT-084
)
mA
1/2
−
MT-035
R2
V
AN-849
SUPPLY
Tutorial, Using Op Amps As
Tutorial, Op Amp Inputs, Outputs,
Application Note, Using Op
V
OUT
ADA4084-2
MT-083
Tutorial,
against
Rev. A | Page 21 of 24
S
,
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 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
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. Therefore, the technique recommended in the
Input 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
extend the dynamic range of the application through the use of
rail-to-rail operation. Referring to the op amp noise model
circuit configuration illustrated in Figure 76, the expression for
an amplifier’s total equivalent input noise voltage for a source
resistance level, R
where:
R
(e
k is the Boltzmann’s constant, 1.38 × 10
T is the ambient temperature in Kelvin of the circuit, 273.15 +
T
(i
power (1 Hz bandwidth).
(e
power (1 Hz bandwidth).
S
A
nOA
nR
nOA
= 2R, the effective, or equivalent, circuit source resistance.
Figure 76. Op Amp Noise Circuit Model Used to Determine Total Circuit
(°C).
)
2
)
ADA4084-2
)
2
e
is the source resistance thermal noise voltage power (4kTR).
2
nT
is the op amp equivalent input noise current spectral
is the op amp equivalent input noise voltage spectral
=
NOISELESS
NOISELESS
( 2
Equivalent Input Noise Voltage and Noise Figure
[
e
R
R
nR
is free from reasonable input voltage range
)
S
2
, is given by
+
(
i
nOA
e
e
nR
nR
×
R
S
e
)
2
nOA
i
i
]
nOA
nOA
+
(
e
nOA
)
2
–23
, units in
J/K.
NOISELESS
OP AMP
ADA4084-2
R
IDEAL
ADA4084-2
S
= 2R
V
Hz