OP484FSZ datasheet(15/24 Pages) AD

15 / 24 page

OP184/OP284/OP484

Rev. D | Page 15 of 24

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INPUT VOLTAGE (V)

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. 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 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.

Figure 44, the OP284 does not have

any internal current limiting resistors; thus, fault currents can

quickly rise to damaging levels.

DESIGNING LOW NOISE CIRCUITS IN SINGLE-

SUPPLY APPLICATIONS

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.

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.

Figure 47 illustrates a typical

noninverting configuration for an overvoltage-protected

amplifier where the series resistance, R , is chosen such that

()

SUPPLY

MAX

−

Referring to the op amp noise model circuit configuration

illustrated in

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.

Figure 48, the expression for an amplifier’s total

equivalent input noise voltage for a source resistance level, RS,

is given by

( )

(

)

[

] ()2

nOA

, units in

where:

VIN

VOUT

1/2

OP284

Figure 47. Resistance in Series with Input Limits Overvoltage Currents

to Safe Values

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 +

T (°C).

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