AD8045ACPZ-R2 datasheet(19/24 Pages) AD

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Data Sheet

AD8045

Rev. C | Page 19 of 24

APPLICATIONS INFORMATION

LOW DISTORTION PINOUT

The AD8045 LFCSP features an Analog Devices new low

distortion pinout. The new pinout provides two advantages

over the traditional pinout. First, improved second harmonic

distortion performance, which is accomplished by the physical

separation of the noninverting input pin and the negative power

supply pin. Second, the simplification of the layout due to the

dedicated feedback pin and easy routing of the gain set resistor

back to the inverting input pin. This allows a compact layout,

which helps to minimize parasitics and increase stability.

The traditional SOIC pinout has been slightly modified as well

to incorporate a dedicated feedback pin. Pin 1, previously a no

connect pin on the amplifier, is now a dedicated feedback pin. The

new pinout reduces parasitics and simplifies the board layout.

Existing applications that use the traditional SOIC pinout can

take full advantage of the outstanding performance offered by

the AD8045. An electrical insulator may be required if the SOIC

rests on the ground plane or other metal trace. This is covered

in more detail in the Exposed Paddle section of this data sheet.

In existing designs, which have Pin 1 tied to ground or to another

potential, simply lift Pin 1 of the AD8045 or remove the potential

on the Pin 1 solder pad. The designer does not need to use the

dedicated feedback pin to provide feedback for the AD8045.

The output pin of the AD8045 can still be used to provide

feedback to the inverting input of the AD8045.

HIGH SPEED ADC DRIVER

When used as an ADC driver, the AD8045 offers results

comparable to transformers in distortion performance.

Many ADC applications require that the analog input signal

be dc-coupled and operate over a wide frequency range. Under

these requirements, operational amplifiers are very effective

interfaces to ADCs. An op amp interface provides the ability to

amplify and level shift the input signal to be compatible with

the input range of the ADC. Unlike transformers, operational

amplifiers can be operated over a wide frequency range down to

and including dc.

Figure 67 shows the AD8045 as a dc-coupled differential driver

for the AD9244, a 14-bit 65 MSPS ADC. The two amplifiers are

configured in noninverting and inverting modes. Both amplifiers

are set with a noise gain of +2 to provide better bandwidth

matching. The inverting amplifier is set for a gain of –1, while

the noninverting is set for a gain of +2. The noninverting input

is divided by 2 in order to normalize its output and make it

equal to the inverting output.

This dc-coupled differential driver is best suited for ±5 V

operation in which optimum distortion performance is

required and the input signal is ground referenced.

VINA

VINB

CML

AD9244

511

Ω

511

Ω

511

Ω

511

Ω

511

Ω

511

Ω

VIN

VCML – VIN

AD8045

0.1

µF

20pF

µF

0.1

µF

OP27

511

Ω

511

Ω

VCML + VIN

Ω

2.5k

Ω

100

Ω

Figure 67. High Speed ADC Driver

The outputs of the AD8045 devices are centered about the 2.5 V

common-mode range of the AD9244. The common-mode

reference voltage from the AD9244 is buffered and filtered via

the OP27 and fed to the noninverting resistor network used in

the level shifting circuit.

The spurious-free dynamic range (SFDR) performance is shown

in Figure 68. Figure 69 shows a 50 MHz single-tone FFT

performance.

INPUT FREQUENCY (MHz)

100

120

100

AD8045

Figure 68. SFDR vs. Frequency

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