Figure 1 – Functional Diagram
D0–D7
8
Composite
Video Controls
CONV
CONV
Feedthrough
2
Decoding
Logic
Data
Registers
Current
Sources
and
Switches
Out +
Out –
4
Current
Source
Biasing
Bandgap
Reference
Amp
+
–
V
EE
V
CC
Ref Out
I
Set
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 V
EE
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
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
