AD9042

–12–

REV. A

If no TTL source is available, a clean sine wave may be

substituted. In the case of the sine source, the matching net-

work is shown below. Since the matching transformer specified

is a 1:1 impedance ratio, R, the load resistor should be selected

to match the source impedance. The input impedance of the

AD9042 is negligible in most cases.

ENCODE

ENCODE

AD9042

R

T1-1T

SINE

SOURCE

Figure 30. Sine Source – Differential Encode

If a low jitter ECL clock is available, another option is to ac-

couple a differential ECL signal to the encode input pins as

shown below. The capacitors shown here should be chip

capacitors but do not need to be of the low inductance variety.

ENCODE

ENCODE

AD9042

ECL

GATE

0.1µF

0.1µF

510

Ω

510

Ω

Figure 31. Differential ECL for Encode

As a final alternative, the ECL gate may be replaced by an ECL

comparator. The input to the comparator could then be a logic

signal or a sine signal.

ENCODE

ENCODE

AD9042

0.1µF

0.1µF

50

Ω

AD96687 (1/2)

510

Ω

510

Ω

Figure 32. ECL Comparator for Encode

Care should be taken not to overdrive the encode input pin

when ac coupled. Although the input circuitry is electrically

protected from over or under voltage conditions, improper

circuit operations may result from overdriving the encode input

pins.

Driving the Analog Input

Because the AD9042 operates off of a single +5 V supply, the

analog input range is offset from ground by 2.4 volts. The

analog input, AIN, is an operational amplifier configured in an

inverting mode (ref. Equivalent Circuits: Analog Input Stage).

2.4 V Reference). Since the operational amplifier forces its

inputs to the same voltage, the inverting input is also at 2.4 volts.

Therefore, the analog input has a Thevenin equivalent of 250 ohms

in series with a 2.4 volt source. It is strongly recommended

that the AD9042’s internal voltage reference be used for the

amplifier offset; this reference is designed to track internal cir-

cuit shifts over temperature.

AD9042

250

Ω

250

Ω

+2.4V

REFERENCE

AIN

THROUGH

50 OHMS

TIED TO

50

Ω

0.1µF

Figure 33. Analog Input Offset by +2.4 V Reference

Although the AD9042 may be used in many applications, it was

specifically designed for communications systems which must

digitize wide signal bandwidths. As such, the analog input was

designed to be ac-coupled. Since most communications products

do not down-convert to dc, this should not pose a problem. One

example of a typical analog input circuit is shown below. In this

application, the analog input is coupled with a high quality chip

capacitor, the value of which can be chosen to provide a low

frequency cutoff that is consistent with the signal being

sampled; in most cases, a 0.1

µF chip capacitor will work well.

AD9042

0.1µF

50

Ω

ANALOG

SIGNAL

SOURCE

0.1µF

AIN

Figure 34. AC-Coupled Analog Input Signal

Another option for ac-coupling is a transformer. The imped-

ance ratio and frequency characteristics of the transformer are

determined by examining the characteristics of the input signal

source (transformer primary connection), and the AD9042 in-

should be chosen to satisfy termination requirements of the

source, given the transformer turns ratio. A blocking capacitor

is required to prevent AD9042 dc bias currents from flowing

through the transformer.

BPF

XFMR

LO

ANALOG

SIGNAL

SOURCE

0.1µF

50

Ω

0.1µF

AD9042

AIN

Figure 35. Transformer-Coupled Analog Input Signal

When calculating the proper termination resistor, note that the

external load resistor is in parallel with the AD9042 analog

input resistance, 250 ohms. The external resistor value can be

calculated from the following equation:

R

1

1

Z

−

1

250

where Z is desired impedance.