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SPT1019

8-BIT, HIGH SPEED D/A CONVERTER

Signal Processing Technologies 

POWER CONSIDERATIONS

The SPT1019 operates from a single -5.2 V standard supply.

Proper bypassing of the supplies will augment the SPT1019’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 SPT1019 has two analog (VEE) power supply pins. Both

supply pins should be properly bypassed as mentioned

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 supply pins while the supply

pins (VEE) become the ground pins. The relative polarities of

the other input and output voltages must be maintained.

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 (ISet), the full-scale

output may be adjusted by varying the reference current. ISET

is controlled through the ISet input on the SPT1019. A method

and equations to set ISet is shown in Figure 2. The SPT1019

uses its own reference voltage for setting up ISet as shown in

Figure 2. The value for ISet 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 ISet is increased 50% more than ISet

for doubly terminated 75 Ohm video applications.

COMPENSATION

REFERENCE CONSIDERATIONS

The SPT1019 has one reference input (ISet) and one refer-

ence output (Ref Out). These pins are connected to the

inverting and noninverting inputs of an internal amplifier that

serves as a reference buffer amplifier. The SPT1019 has a

bandgap reference connected internally to the inverting input

of the buffer amplifier and Ref Out.

The SPT1019 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 VEE 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.

Figure 3 - Timing Diagram

CONV

-1.3 V

CONV

-1.3 V

OUT -

OUT +

tPWH

tPWL

tS

tH

Data Control

Inputs

tDST

tDSC

tSI

1/2 LSB

1/2 LSB

SPT

6

SPT1019

5/14/97

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