AD8352

Data Sheet

Rev. C | Page 14 of 19

48

38

0

200

FREQUENCY (MHz)

47

46

45

44

43

42

41

40

39

50

100

150

6dB

10dB

15dB

18dB

Figure 32. Third-Order Intermodulation Distortion, OIP3 vs.

Frequency for Various Gain Settings

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

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 34 and Figure 35 (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 Single-Ended Input Operation

section. The 100 MHz FFT plots shown in Figure 36 and Figure 37

display the results for the differential configuration. Though not

shown, the single-ended, third-order levels are similar.

The 50 Ω resistor shown in Figure 34 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 35 represents a single-

ended input to differential output configuration for driving the

200 Ω) provide the input impedance match for a 50 Ω system.

Again, if input reflections are minimal, this impedance match is

the output voltages that are required for second-order distortion

The AD9445 presents approximately 2 kΩ in parallel with

5 pF/differential load to the AD8352 and requires a 2.0 V p-p

full-scale output operation.

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. However, at frequencies above ~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

appreciably 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, with

order intermodulation levels shown in these figures.

Placing antialiasing filters between the ADC and the amplifier

is a common approach for improving overall noise and broad-

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

applications. 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.