SPT
5
3/28/00
SPT5140
OUTPUT CONSIDERATIONS
The analog outputs are designed to directly drive a dou-
bly terminated 50 or 75 ohm load transmission system as
shown. The source impedances of the SPT5140 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.
POWER CONSIDERATIONS
The SPT5140 operates from a single –5.2 V standard
supply. Proper bypassing of the supplies will augment the
SPT5140’s inherent supply noise rejection characteris-
tics. As shown in figure 2, each supply pin should be by-
passed as close to the device as possible with 0.01
µF
and 10
µF capacitors.
The SPT5140 has two analog (VEE) power supply pins.
Both supply pins should be properly bypassed as men-
tioned previously. This device also has two analog (VCC)
ground pins. Both ground pins should be tied to the
analog ground plane. Power and ground pins must be
connected in all applications. If a +5 V power source is
required, the ground pins (VCC) become the positive sup-
ply pins while the supply pins (VEE) become the ground
pins. The relative polarities of the other input and output
voltages must be maintained.
Figure 2 – Typical Interface Circuit
10.
= VCC = AGND
See figure 9 for detail on Ref Buffer.
R2
1 k
Ω
4 to 15
Decode
Video
Control
Inputs
Video
Data
Inputs
Clock
4
4
)
(
Ref
Buffer
V+
R1
1 k
Ω
ISet
ISet
*
Bandgap
Reference
V–
Ref In
1k
Ω
Optional External
Reference
–5.2 V
.01
µF
Remove J1 For
External Reference
LM113/313
FT
FH
Blank
BRT
Sync
D0 (LSB)
D1
D2
D3
D4
D5
D6
D7 (MSB)
CONV
CONV
2 k
Ω
Video Monitor
Out+
R3
50/75
Ω
R4
50/75
Ω
50/75
Ω COAX
All reference resistors 1/8 W 1% metal
film, power supply decoupling 50 V
ceramic disc
NOTES:
V– = –1.2 V (typical) for LM113 or VBG
V+ = –1.2 V
RL = R3 / / R4
K = 15.8069
K1 = 1.7617
K2 = 10.0392
FB = Ferrite bead, Fair-rite pin 217430011
or equivalent.
1.
= ECL Termination
x
2.
3.
4.
6.
7.
8.
9.
ISet =
V+
αT(R
1+R2)
; typ = 1.105 mA
VOUT =
255
5.
VSync = (K x ISet x RL) + (K2 x ISet x RL)
*An external reference can be used or the reference from
Ref Out can drive three SPT5140s.
Out–
255–Digital Input Code
xISet RL +
[K
1 x ISet x RL(bright)
K
VEE
VCC
–5.2 V
.01
µF
10
µF
FB
= DGND (digital input drivers).
12.
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11.
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