AD8045ACP-R2 datasheet(19/24 Pages) AD

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AD8045

Rev. A | Page 19 of 24

APPLICATIONS

LOW DISTORTION PINOUT

The AD8045 LFCSP package features 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 pro-

vide feedback to the inverting input of the AD8045.

HIGH SPEED ADC DRIVER

When used as an ADC driver, the AD8045 offers results compa-

rable 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 require-

ments, 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 amplifi-

ers 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 AD8045s are centered about the AD9244’s

common-mode range of 2.5 V. 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

110

120

100

AD8045

Figure 68. SFDR vs. Frequency

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