8

5/9/01

SPT7720

REFERENCES

To save on parts count, design time, and PC board real

estate, the SPT7720 utilizes an internal reference. No

other external components are required to implement this

feature.

VOLTAGE REFERENCE CIRCUIT

The SPT7720 has an on-board voltage reference circuit

typically. The circuit is commonly used to drive the center

tap of the RF transformer in fully differential applications.

For single-ended applications, this output can be used to

provide the level shifting required for the single-to-differen-

tial converter conversion circuit.

ENCODE INPUT

The ENCODE input on the SPT7720 can be driven by ei-

ther a single-ended or differential clock circuit and can

handle TTL, PECL, and CMOS signals. When operating at

high sample rates it is important to keep the pulse width of

the duty signal as close to 50% as possible. For TTL/

CMOS single-ended ENCODE inputs, the rise time of the

signal also becomes an important consideration. The

ENCODE input is 300

Ω into a bipolar differential pair.

ENCODE

is internally biased to 1.5 V with a Thevenin

equivalent of 5.25 k

Ω.

DIGITAL INPUTS

The DS input is 35

Ω into one side of a differential pair.

5.25 k

Ω.

pair. The other side is internally biased to 1.5 V and does

not connect to the outside.

DIGITAL OUTPUTS

The output circuitry of the SPT7720 has been designed to

be able to support two separate output modes. The

demuxed (double-wide) mode supports interleaved data

output. The single-channel mode is not demuxed and can

support direct output at speeds up to 100 MSPS.

The output format is straight binary (table I).

Table I – Output Data Format

Analog Input

Output Code

D7–D0

+FS

1111 1111

+FS – 1/2 LSB

1111 111Ø

+1/2 FS

ØØØØ ØØØØ

–FS + 1/2 LSB

0000 000Ø

–FS

0000 0000

Ø indicates the flickering bit between logic 0 and 1

42). Table II describes the mode switching options.

Table II – Output Data Modes

Output Mode

DEMUX

Interleaved Dual Channel Output

0

Single Channel Data Output

(Bank A only 100 MSPS max)

1

EVALUATION BOARD

The EB7720 evaluation board is available to aid designers

in demonstrating the full performance of the SPT7720.

This board includes a clock driver and reset circuit, adjust-

able references and common mode, a single-ended to dif-

ferential input buffer and a single-ended to differential

transformer (1:1). An application note (AN7720) describ-

ing the operation of this board, as well as information on

the testing of the SPT7720, is also available. Contact the

factory for price and availability of the EB7720.