10-Bit, 105Msps, Single +3.3V, Low-Power

ADC with Internal Reference

______________________________________________________________________________________

13

Using Transformer Coupling

An RF transformer (Figure 8) provides an excellent

solution to convert a single-ended source signal to a

fully differential signal, required by the MAX1449 for

optimum performance. Connecting the center tap of the

the input. Although a 1:1 transformer is shown, a step-

up transformer may be selected to reduce the drive

requirements. A reduced signal swing from the input

driver, such as an op amp, may also improve the over-

all distortion.

In general, the MAX1449 provides better SFDR and

THD with fully differential input signals than single-

ended drive, especially for very high input frequencies.

In differential input mode, even-order harmonics are

lower as both inputs (IN+, IN-) are balanced, and each

of the inputs only requires half the signal swing com-

pared to single-ended mode.

Single-Ended AC-Coupled Input Signal

Figure 9 shows an AC-coupled, single-ended applica-

tion. The MAX4108 op amp provides high speed, high

bandwidth, low-noise, and low-distortion to maintain the

integrity of the input signal.

Grounding, Bypassing

and Board Layout

The MAX1449 requires high-speed board layout design

techniques. Locate all bypass capacitors as close to

the device as possible, preferably on the same side as

the ADC, using surface mount devices for minimum

two parallel 0.1µF ceramic capacitors and a 2.2µF

bipolar capacitor to GND. Follow the same rules to

boards with separated ground and power planes pro-

duce the highest level of signal integrity. Consider

OUTPUT

DATA D9–D0

OE

HIGH-Z

HIGH-Z

VALID DATA

Figure 5. Output Enable Timing

N - 6

N

N - 5

N + 1

N - 4

N + 2

N - 3

N + 3

N - 2

N + 4

N - 1

N + 5

N

N + 6

N + 1

5.5 CLOCK-CYCLE LATENCY

ANALOG INPUT

CLOCK INPUT

DATA OUTPUT

Figure 6. System and Output Timing Diagram