AD8614/AD8644

Rev. B | Page 9 of 16

THEORY OF OPERATION

The AD8614/AD8644 are processed using Analog Devices high

voltage, extra fast complementary bipolar (HV XFCB) process.

This process includes trench-isolated transistors that lower

parasitic capacitance.

Figure 26 shows a simplified schematic of the AD8614/AD8644.

The input stage is rail-to-rail, consisting of two complementary

differential pairs, one NPN pair and one PNP pair. The input

stage is protected against avalanche breakdown by two back-to-

back diodes. Each input has a 1.5 kΩ resistor that limits input

current during overvoltage events and furnishes phase reversal

protection if the inputs are exceeded. The two differential pairs

are connected to a double-folded cascode. This is the stage in

the amplifier with the most gain. The double-folded cascode

differentially feeds the output stage circuitry. Two complemen-

tary common emitter transistors are used as the output stage.

This allows the output to swing to within 125 mV from each rail

with a 10 mA load. The gain of the output stage, and thus the

open-loop gain of the op amp, depends on the load resistance.

The AD8614/AD8644 have no built-in short-circuit protection.

The short-circuit limit is a function of high current roll-off of

the output stage transistors and the voltage drop over the

resistor shown on the schematic at the output stage. The voltage

over this resistor is clamped to one diode during short-circuit

voltage events.

OUTPUT SHORT-CIRCUIT PROTECTION

To achieve a wide bandwidth and high slew rate, the output of

the AD8614/AD8644 is not short-circuit protected. Shorting

the output directly to ground or to a supply rail can destroy the

device. The typical maximum safe output current is 70 mA.

In applications where some output current protection is needed,

but not at the expense of reduced output voltage headroom, a

low value resistor in series with the output can be used. This is

shown in Figure 25. The resistor is connected within the

exceed 70 mA.

For 18 V single-supply applications, resistors less than 261 Ω are

not recommended.

AD86x4

261Ω

18V

Figure 25. Output Short-Circuit Protection

–

+

1.5kΩ

1.5kΩ

Figure 26. Simplified Schematic