Data Sheet

ADA4084-2

Rev. A | Page 21 of 24

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 ADA4084-2 does not have any internal current limiting resis-

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

is chosen, such that

(

)

mA

5

SUPPLY

MAX

IN

S

V

V

R

−

=

For example, a 1 kΩ resistor protects the ADA4084-2 against

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.

R1

R2

1/2

ADA4084-2

Figure 75. Resistance in Series with Input

Limits Overvoltage Currents to Safe Values

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 MT-035 Tutorial, Op Amp Inputs, Outputs,

Single-Supply, and Rail-to-Rail Issues; the MT-083 Tutorial,

Comparators, the MT-084 Tutorial, Using Op Amps As

Comparators; and the AN-849 Application Note, Using Op

Amps as Comparators, at www.analog.com.

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 ADA4084-2 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. 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 ADA4084-2

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

[

]

2

2

2

)

(

)

(

)

(

2

nOA

S

nOA

nR

nT

e

e

e

R

i

+

×

+

=

, units in

Hz

V

where:

T is the ambient temperature in Kelvin of the circuit, 273.15 +

power (1 Hz bandwidth).

power (1 Hz bandwidth).

R

NOISELESS

R

NOISELESS

IDEAL

NOISELESS

OP AMP

Figure 76. Op Amp Noise Circuit Model Used to Determine Total Circuit

Equivalent Input Noise Voltage and Noise Figure