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SPT5140

8-BIT, ULTRAHIGH-SPEED D/A CONVERTER

Cadeka Microcircuits LLC. 

Figure 1 – Functional Diagram

D0–D7

Composite

Video Controls

CONV

CONV

Feedthrough

8

Decoding

Logic

4

2

Data

Registers

Current

Sources

and

Switches

Current

Source

Biasing

Out +

Out –

Bandgap

Reference

Amp

+

–

VEE

VCC

Ref Out

ISet Ref In

APPLICATION INFORMATION

The SPT5140 is a high-speed video digital-to-analog

converter capable of up to 400 MWPS conversion rates.

This makes the devices suitable for driving 2048 X 2048

pixel displays at update rates of 60 to 90 Hz.

In addition, the SPT5140 includes an internal bandgap

reference which may be used to drive two other

SPT5140s if desired.

The SPT5140 has 10KH and 100K ECL logic level com-

patible video control and data inputs. The complementary

analog output currents produced by the devices are pro-

portional to the product of the digital control and data

inputs in conjunction with the analog reference current.

The SPT5140 is segmented so that the four MSBs of the

input data are separated into a parallel “thermometer”

code. From here, fifteen current sinks, which are identi-

cal, are driven to fabricate sixteen coarse output levels.

The remaining four LSBs drive four binary weighted

current switches.

MSB currents are then summed with the LSBs that pro-

vide a one-sixteenth of full-scale contribution to provide

the 256 distinct analog output levels.

The video control inputs drive weighted current sinks

which are added to the output current to produce com-

posite video output levels. These controls — Sync, Blank,

Reference White (Force High) and Bright — are needed

in video applications.

Another feature that similar video D/A converters do not

have is the Feedthrough Control. This pin allows regis-

tered or unregistered operation of the video control and

data inputs. In the registered mode, the composite func-

tions are latched to the pixel data to prevent screen-edge

distortions generally found on unregistered video DACs.

TYPICAL INTERFACE CIRCUIT

GENERAL

A typical interface circuit using the SPT5140 in a color

raster application is shown in figure 2. The SPT5140

requires few external components and is extremely easy

to use. The very high operating speeds of the SPT5140

require good circuit layout, decoupling of supplies, and

proper design of transmission lines. The following consid-

erations should be noted to achieve best performance.

INPUT CONSIDERATIONS

Video input data and controls may be directly connected

to the SPT5140. Note that all ECL inputs are terminated

as closely to the device as possible to reduce ringing,

crosstalk and reflections. A convenient and commonly

used microstrip impedance is about 130 ohms, which is

easily terminated using a 330 ohm resistor to VEE and a

220 ohm resistor to ground. This arrangement gives a

Thevenin equivalent termination of 130 ohms to –2 volts

without the need for a –2 volt supply. Standard SIP

(Single Inline Package) 220/330 resistor networks are

available for this purpose.

It is recommended that stripline or microstrip techniques

be used for all ECL interface. Printed circuit wiring

of known impedance over a solid ground plane is

recommended.

SPT5140

4

3/28/00

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