AD8065WARTZ-R7 datasheet(21/29 Pages) AD

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AD8065/AD8066

Rev. J | Page 20 of 28

THEORY OF OPERATION

The AD8065/AD8066 are voltage feedback operational amplifiers

that combine a laser-trimmed JFET input stage with the Analog

Devices eXtra Fast Complementary Bipolar (XFCB) process,

resulting in an outstanding combination of precision and speed.

The supply voltage range is from 5 V to 24 V. The amplifiers feature

a patented rail-to-rail output stage capable of driving within 0.5 V

of either power supply while sourcing or sinking up to 30 mA.

Also featured is a single-supply input stage that handles common-

mode signals from below the negative supply to within 3 V of the

positive rail. Operation beyond the JFET input range is possible

because of an auxiliary bipolar input stage that functions with

input voltages up to the positive supply. The amplifiers operate as

if they have a rail-to-rail input and exhibit no phase reversal

behavior for common-mode voltages within the power supply.

With voltage noise of 7 nV/√Hz and −88 dBc distortion for

1 MHz, 2 V p-p signals, the AD8065/AD8066 are a great choice

for high resolution data acquisition systems. Their low noise,

sub-pA input current, precision offset, and high speed make

them superb preamps for fast photodiode applications. The

speed and output drive capability of the AD8065/AD8066 also

make them useful in video applications.

CLOSED-LOOP FREQUENCY RESPONSE

The AD8065/AD8066 are classic voltage feedback amplifiers

with an open-loop frequency response that can be approximated as

the integrator response shown in Figure 53. Basic closed-loop

frequency response for inverting and noninverting configurations

can be derived from the schematics shown.

NONINVERTING CLOSED-LOOP FREQUENCY

RESPONSE

Solving for the transfer function

()

crossover

where fcrossover is the frequency where the amplifier’s open-loop

gain equals 0 db

At dc

Closed-loop −3 dB frequency

crossover

3dB

−

INVERTING CLOSED-LOOP FREQUENCY

RESPONSE

()

crossover

−

At dc

−

Closed-loop −3 dB frequency

crossover

−3

FREQUENCY (MHz)

0.01

100

0.1

fcrossover = 65MHz

A = (2

π× fcrossover)/s

Figure 53. Open-Loop Gain vs. Frequency and Basic Connections

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