AD8614/AD8644

–5–

REV. 0

SUPPLY VOLTAGE – V

0

220

46

8

10

12

14 16

18

9

8

0

4

3

2

1

7

5

6

SR+

SR

Figure 19. Slew Rate vs. Supply

Voltage

FREQUENCY – Hz

100

10

1

10

100

10k

1k

Figure 20. Voltage Noise Density

vs. Frequency

FREQUENCY – Hz

100

10

1

10

100

10k

1k

Figure 21. Voltage Noise Density vs.

Frequency

APPLICATIONS SECTION

Theory of Operation

The AD8614/AD8644 are processed using Analog Devices’ high

voltage, high speed, complementary bipolar process—HV XFCB.

This process includes trench isolated transistors that lower parasitic

capacitance.

Figure 22 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 k

Ω resistor that limits input current

during over-voltage 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 complementary common emitter tran-

sistors 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.

+

1.5k

1.5k

Figure 22. Simplified Schematic

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 may 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 23. The resistor is connected within the feedback loop

swings up to 18 V, the output current will not exceed 70 mA.

For 18 V single supply applications, resistors less than 261

Ω are

not recommended.