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SPT1019AIN

8-BIT, HIGH SPEED D/A CONVERTER

Signal Processing Technologies 

Figure 2 - Typical Interface Circuit

Video Monitor

FT

FH

4

Video

Control

Inputs

Blank

BRT

Sync

4

Video

Data

Inputs

Clock

D0 (LSB)

D1

D2

D3

D4

D5

D6

D7 (MSB)

CONV

CONV

4 To 15

Decode

R1

500 Ω

R2

750 Ω

V+ ISet

ISet

.01 µF

Ref Out

FB

Ref

Buffer

Bandgap

Reference

VCC

VEE

.01 µF

.01 µF

C Comp

10 µF

-5.2 V

Out-

R3

50/75 Ω

Out+

50/75 Ω COAX

R4

50/75 Ω

NOTES:

1. V- = -1.2 V (typical) for LM113.

2. V+ = -1.2 V

3.

ISet

=

V+

αT(R1+R2)

4. RL = R3 / / R4

5.

VOut-

=

K

255-Digital Input

255

Code

x I Set

RL + KI x I Set x R L(Bright)

6. VSync = (K x ISet x RL) + (K2 x ISet x RL)

K = 15.8069

K1 = 1.7617

K2 = 10.0392

7. FB = Ferrite bead, Fair-rite P/N 217430011 or equivalent.

COMP

8. All reference resistors 1/8 W 1% metal film power.

supply decoupling 50 V ceramic disc.

9. x = ECL termination.

10.

= VCC = AGND

11 See figure 8 for detail on Ref Buffer.

.

12. Ref Out on the SPT1019 can be used to drive up to

two SPT1018 reference inputs.

13. = DGND (digital input drivers).

TYPICAL INTERFACE CIRCUIT

GENERAL

A typical interface circuit using the SPT1019 in a color raster

application is shown in figure 2. The SPT1019 requires few

external components and is extremely easy to use. The very

high operating speeds of the SPT1019 require good circuit

layout, decoupling of supplies, and proper design of trans-

mission lines. The following considerations should be noted

to achieve best performance.

INPUT CONSIDERATIONS

Video input data and controls may be directly connected to

the SPT1019. 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 ter-

mination 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 recom-

mended that stripline or microstrip techniques be used for all

ECL interface. Printed circuit wiring of known impedance

over a solid ground plane is recommended.

OUTPUT CONSIDERATIONS

The analog outputs are designed to directly drive a dual 50 or

75 ohm load transmission system as shown. The source

impedances of the SPT1019 outputs are high impedance

current sinks. The load impedance (RL) must be 25

or 37.5 ohms to attain standard RS-343-A video levels. Any

deviation from this impedance will affect the resulting video

output levels proportionally. As with the data interface, it is

important that the analog transmission lines have matched

impedance throughout, including connectors and transitions

between printed wiring and coaxial cable. The combination of

matched source termination resistor RS and load terminator

RL minimizes reflections of both forward and reverse travel-

ing waves in the analog transmission system. The return path

for analog output current is VCC which is connected to the

source termination resistor RS.

SPT

5

SPT1019

5/14/97

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