AD9054A

–13–

REV. B

Digital Inputs

SNR performance is directly related to the sampling clock sta-

bility in A/D converters, particularly for high input frequencies

and wide bandwidths. A low jitter clock (<10 ps @ 100 MHz)

is essential for optimum performance when digitizing signals

that are not presampled.

ENCODE and Data Select (DS) can be driven differentially or

single-ended. For single-ended operation, the complement

inputs (

ENCODE, DS) are internally biased to V

by a high impedance on-chip resistor divider (Figure 5), but

they may be externally driven to establish an alternate threshold

if desired. A 0.1

µF decoupling capacitor to ground is sufficient

to maintain a threshold appropriate for TTL or CMOS logic.

When driven differentially, ENCODE and DS will accommo-

date differential signals centered between 1.5 V and 4.5 V with

a total differential swing

Note the 6-diode clock input protection circuitry in Figure 5.

This limits the differential input voltage to ~

±2.1 V. When the

diodes turn on, current is limited by the 300

Ω series resistor.

Exceeding 2.1 V across the differential inputs will have no im-

pact on the performance of the converter, but be aware of the

clock signal distortion that may be produced by the nonlinear

impedance at the converter.

CLOCK

CLOCK

ENC

ENC

CLOCK

ENC

ENC

0.1 F

a. Driving Differential Inputs Differentially

b. Driving Differential Inputs Single-Endedly

Figure 34. Input Signal Level Definitions

Single Port Mode

When operated in a Single Port mode (

DEMUX = HIGH), the

timing of the AD9054A is similar to any high speed A/D Con-

verter (Figure 1).

A sample is taken on every rising edge of ENCODE, and the

resulting data is produced on the output pins following the

FOURTH rising edge of ENCODE after the sample was taken

the next rising edge of ENCODE.

The maximum clock rate is specified as 100 MSPS. This is

recommended because the guaranteed output data valid time

4.8 ns at 100 MHz. This is about as fast as standard logic is able

to capture the data with reasonable design margins. The AD9054A

will operate faster in single-channel mode if you are able to

capture the data.

When operating in Single-Channel Mode, the outputs at Port B

are held static in a random state.

Figure 35 shows the AD9054A used in single-channel output

mode. The analog input (

±0.5 V) is ac coupled and the ENCODE

input is driven by a TTL level signal. The chip’s internal refer-

ence is used.

VIN

0.1 F

+5V

1k

0.1 F

0.1 F

NC

CLOCK

VREF OUT

VREF IN

AIN

AIN

DEMUX

AD9054A

DS

DS ENC ENC

A PORT

NC = NO CONNECT

Figure 35. Single Port Mode—AC-Coupled Input—Single-

Ended Encode

Dual Port Mode

In Dual Port Mode (

DEMUX = LOW), the conversion results

are alternated between the two output ports (Figure 2). This

limits the data output rate at either port to 1/2 the conversion

rate (ENCODE), and supports conversion at up to 200 MSPS

with TTL/CMOS compatible interfaces. Dual Channel Mode is

required for guaranteed operation above 100 MSPS, but may be

enabled at any specified conversion rate.

The multiplexing is controlled internally via a clock divider,

which introduces a degree of ambiguity in the port assignments.

Figure 2 illustrates that, prior to synchronization, either Port A

or Port B may produce the even or odd samples. This is re-

solved by exercising the Data Sync (DS) control, a differential

input (identical to the ENCODE input), which facilitates opera-

tion at high speed.

At least once after power-up, and prior to using the conversion

data, the part needs to be synchronized by a falling edge (or a

positive-going pulse) on DS (observing setup and hold times

with respect to ENCODE). If the converter’s internal timing is

in conflict with the DS signal when it is exercised, then two data

samples (one on each port) are corrupted as the converter is

resynchronized. The converter then produces data with a

known phase relationship from that point forward.

Note that if the converter is already properly synchronized, the

DS pulse has no effect on the output data. This allows the con-

verter to be continuously resynchronized by a pulse at 1/2 the

ENCODE rate. This signal is often available within a system, as

it represents the master clock rate for the demultiplexed output

data. Of course, a single DS signal may be used to synchronize

multiple A/D converters in a multichannel system.