POWER CONSIDERATIONS
The SPT1018 operates from a single -5.2 V standard supply.
Proper bypassing of the supplies will augment the SPT1018’s
inherent supply noise rejection characteristics. As shown in
figure 2, each supply pin should be bypassed as close to the
device as possible with 0.01
µF
and 10
µF
capacitors.
The SPT1018 has two analog (V
EE
) power supply pins. Both
supply pins should be properly bypassed as mentioned
previously. This device also has two analog (V
CC
) 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 (V
CC
) become the positive supply pins while the supply
pins (V
EE
) become the ground pins. The relative polarities of
the other input and output voltages must be maintained.
REFERENCE CONSIDERATIONS
The SPT1018 has two reference inputs: Ref+ and Ref-. These
pins are connected to the inverting and noninverting inputs of
an internal amplifier that serves as a reference buffer amplifier.
The output of the buffer amplifier is the reference for the
current sinks. The amplifier feedback loop is connected
around one of the current sinks to achieve better accuracy.
(See figure 8.)
Since the analog output currents are proportional to the digital
input data and the reference current (I
Set
), the full-scale
output may be adjusted by varying the reference current. I
Set
is controlled through the Ref+ input on the SPT1018. A
method and equations to set I
Set
is shown in figure 2. The
Figure 3 - Timing Diagram
SPT1018 uses an external negative voltage reference. The
external reference must be stable to achieve a satisfactory
output and the Ref- pin should be driven through a resistor to
minimize offsets caused by bias current. The value for I
Set
can be varied with the 500 ohm trimmer to change the full
scale output. A double 50 ohm load (25 ohm) can be driven
if I
Set
is increased 50% more than I
Set
for doubly terminated
75 ohm video applications.
COMPENSATION
The SPT1018 provides an external compensation input
(COMP) for the reference buffer amplifier. In order to use this
pin correctly, a capacitor should be connected between
COMP and V
EE
as shown in figure 2. Keep the lead lengths
as short as possible. If the reference is to be kept as a
constant, use a large capacitor (.01
µF).
The value of the
capacitor determines the bandwidth of the amplifier. If modu-
lation of the reference is required, smaller values of capaci-
tance can be used to achieve up to a 1
MHz
bandwidth.
DATA INPUTS AND VIDEO CONTROLS
The SPT1018 has standard single-ended data inputs. The
inputs are registered to produce the lowest differential data
propagation delay (skew) to minimize glitching. There are
also four video control inputs to generate composite video
outputs. These are Sync, Blank, Bright and Reference White
or Force High. Also provided is the Feedthrough control as
mentioned earlier. The controls and data inputs are all 10 KH
and 100K ECL compatible. In addition, all have internal
pulldown resistors to leave them at a logic low so the pins are
inactive when not used. This is useful if the devices are
applied as standard DACs without the need for video controls
or if less than eight bits are used.
t
PWH
CONV
-1.3 V
CONV
t
PWL
t
S
t
H
-1.3 V
Data Control
Inputs
t
DST
t
DSC
1/2 LSB
OUT -
OUT +
t
SI
1/2 LSB
SPT1018
6
5/14/97
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
