AD8622/AD8624

Rev. C | Page 15 of 20

APPLICATIONS INFORMATION

INPUT PROTECTION

The maximum differential input voltage that can be applied to

the AD8622/AD8624 is determined by the internal diodes

connected across its inputs and series resistors at each input. These

internal diodes and series resistors limit the maximum

differential input voltage to ±10 V and are needed to prevent base-

emitter junction breakdown from occurring in the input stage of

the AD8622/AD8624 when very large differential voltages are

applied. In addition, the internal resistors limit the currents that

flow through the diodes. However, in applications where large

differential voltages can be inadvertently applied to the device,

large currents may still flow through these diodes. In such a

case, external resistors must be placed at both inputs of the op

amp to limit the input currents to ±10 mA (see Figure 54).

AD862x

500Ω

500Ω

R1

R2

2

3

1

Figure 54. Input Protection

PHASE REVERSAL

An undesired phenomenon, phase reversal (also known as

phase inversion) occurs in many op amps when one or both of

the inputs are driven beyond the specified input voltage range

(IVR), in effect reversing the polarity of the output. In some

cases, phase reversal can induce lockups and even cause

equipment damage as well as self destruction.

The AD8622/AD8624 amplifiers have been carefully designed to

prevent output phase reversal when both inputs are maintained

within the specified input voltage range. In addition, even if one

or both inputs exceed the input voltage range but remain within

the supply rails, the output still does not phase reverse. Figure 55

shows the input/output waveforms of the AD8622/AD8624

configured as a unity-gain buffer with a supply voltage of ±15 V.

TIME (200µs/DIV)

Figure 55. No Phase Reversal

MICROPOWER INSTRUMENTATION AMPLIFIER

The AD8622 is a dual, high precision, rail-to-rail output op amp

operating at just 215 μA quiescent current per amplifier. Its

ultralow offset, offset drift, and voltage noise, combined with its

very low bias current and high common-mode rejection ratio

(CMRR), are ideally suited for high accuracy and micropower

instrumentation amplifier.

Figure 56 shows the classic 2-op-amp instrumentation amplifier

with four resistors using the AD8622. The key to high CMRR

for this instrumentation amplifier are resistors that are well

matched from both the resistive ratio and the relative drift. For

true difference amplification, matching of the resistor ratio is

very important, where R3/R4 = R1/R2. Assuming perfectly

matched resistors, the gain of the circuit is 1 + R2/R1, which is

approximately 100. Tighter matching of two op amps in one

package, like the AD8622, offers a significant boost in

performance over the classical 3-op-amp configuration. Overall,

the circuit only requires about 430 µA of supply current.

+15V

–15V

V2

V1

R1

10.1kΩ

R2

1MΩ

R3

10.1kΩ

R4

1MΩ

NOTES

2. TYPICAL: 0.01mV < |V2 – V1| < 149.7mV

4. USE MATCHED RESISTORS.

1/2

AD8622

+

–

+15V

–15V

1/2

AD8622

+

–

Figure 56. Micropower Instrumentation Amplifier