AD10242TZ 查看數據表（PDF） - Analog Devices

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AD10242TZ

Dual, 12-Bit, 40 MSPS MCM A/D Converter with Analog Input Signal Conditioning

Analog Devices 

AD10242

If a logic threshold other than the nominal 1.6 V is required,

the following equations show how to use an external resistor,

RX, to raise or lower the trip point (see Figure 4, R1 = 17 kΩ,

R2 = 8 kΩ).

V1

=

5R2RX

R1R2 + R1RX +

R2RX

to lower logic threshold.

ENCODE

SOURCE

Vl

0.01µF

RX

ENCODE

ENCODE

+5V

R1

R2

AD10242

Figure 21. Lower Threshold for Encode

V1

=

R2

5R2

+

R1RX

R1+ RX

to raise logic threshold.

AVCC

RX

ENCODE

SOURCE

Vl

0.01µF

ENCODE

ENCODE

+5V

R1

R2

AD10242

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

tuted. In the case of the sine source, the matching network 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.

SINE

SOURCE

T1-1T

ENCODE

R

AD10242

ENCODE

Figure 24. 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 capaci-

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

ECL

GATE

510Ω

0.1µF

0.1µF

510Ω

–VS

ENCODE

AD10242

ENCODE

Figure 22. Raise Logic Threshold for Encode

While the single ended encode will work well for many applica-

tions, driving the encode differentially will provide increased

performance. Depending on circuit layout and system noise, a

1 dB to 3 dB improvement in SNR can be realized. It is recom-

mended that the encode signal be ac-coupled into the ENCODE

and ENCODE pins.

The simplest option is shown below. The low jitter TTL signal

is coupled with a limiting resistor, typically 100 Ω, to the pri-

mary side of an RF transformer (these transformers are inexpen-

sive and readily available; Part No. in figure is from Mini-

Circuits). The secondary side is connected to the ENCODE

and ENCODE pins of the converter. Since both encode inputs

are self-biased, no additional components are required.

100Ω T1-1T

TTL

ENCODE

AD10242

ENCODE

Figure 25. 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.

AD96687 (1/2)

50Ω

510Ω

0.1µF

0.1µF

510Ω

ENCODE

AD10242

ENCODE

–VS

Figure 26. ECL Comparator for Encode

Care should be taken not to overdrive the encode input pin

when ac coupled. Although the input circuitry is electrically pro-

tected from over or under voltage conditions, improper circuit

operations may result from overdriving the encode input pin.

Figure 23. TTL Source—Differential Encode

REV. A

–11–

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