AD8352

Rev. 0| Page 14 of 20

6.0

0

30

190

FREQUENCY (MHz)

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

50

70

90

110

130

150

170

6db

10db

15db

18dB

These AD8352 simplified circuits provide the gain, isolation,

and distortion performance necessary for efficiently driving

high linearity converters such as the AD9445. This device also

provides balanced outputs whether driven differentially or

single-ended, thereby maintaining excellent second-order

distortion levels. Though at frequencies above approximately

100 MHz, due to phase related errors, single-ended, second-

order distortion is relatively higher. The output of the amplifier

is ac-coupled to allow for an optimum common-mode setting at

the ADC input. Input ac-coupling can be required if the source

also requires a common-mode voltage that is outside the

optimum range of the AD8352. A VCM common-mode pin is

provided on the AD8352 that equally shifts both input and

output common-mode levels. Increasing the gain of the

AD8352 increases the system noise and, thus, decreases the

SNR (3.5 dB at 100 MHz input for Av=10 dB) of the AD9445

when no filtering is used. Note that amplifier gains from 3 dB to

affect distortion levels. These circuits, when configured

properly, can result in SFDR performance of better than 87 dBc

at 70 MHz and 82 dBc at 180 MHz input. Single-ended drive,

third-order intermodulation levels shown in these figures.

HIGH PERFORMANCE ADC DRIVING

The AD8352 provides the gain, isolation, and balanced low

distortion output levels for efficiently driving wideband ADCs

such as the AD9445.

Figure 32 and Figure 33 (single and differential input drive)

illustrate the typical front-end circuit interface for the AD8352

differentially driving the AD9445 14-bit ADC at 105 MSPS. The

AD8352, when used in the single-ended configuration shows

little or no degradation in overall third-order harmonic

performance (vs. differential drive). See the

Placing antialiasing filters between the ADC and the amplifier

is a common approach for improving overall noise and broadband

distortion performance for both band-pass and low-pass appli-

cations. For high frequency filtering, matching to the filter is

required. The AD8352 maintains a 100 Ω output impedance

well beyond most applications and is well-suited to drive most

filter configurations with little or no degradation in distortion.

Single-Ended Input

Operation section. The 100 MHz FFT plots shown in Figure 34

and Figure 35 display the results for the differential

configuration. Though not shown, the single-ended third-order

levels are similar.

AD9445

AD8352

IF/RF INPUT

ADT1-1WT

50Ω

0Ω

24Ω

24Ω

0Ω

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

0.1µF

16

1

2

3

4

5

10

11

8, 11

14

Figure 32

The 50 Ω resistor shown in

provides a 50 Ω

differential input impedance to the source for matching

considerations. When the driver is less than one eighth of the

wavelength from the AD8352, impedance matching is not

required thereby negating the need for this termination resistor.

The output 24 Ω resistors provide isolation from the analog-to-

digital input. Refer to the Layout and Transmission Line Effects

section for more information. The circuit in Figure 33

represents a single-ended input to differential output configura-

tion for driving the AD9445. In this case, the input 50 Ω resistor

for a 50 Ω system. Again, if input reflections are minimal, this

required to balance the output voltages that are required for

providing distortion cancellation. The AD9445 presents

approximately 2 kΩ in parallel with 5 pF/differential load to the

between VIN+ and VIN− for a full-scale output operation.

Figure 32. Differential Input to the AD8352 Driving the AD9445

25Ω

50Ω

50Ω

AD8352

33Ω

33Ω

0.1µF

0.1µF

0.1µF

0.1µF

AC

VIP

VIN

VOP

VON

200Ω

AD9445

VIN+

VIN–

Figure 33. Single-Ended Input to the AD8352 Driving the AD9445