SPT1018 데이터 시트보기 (PDF) - Signal Processing Technologies

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SPT1018

8-BIT, HIGH SPEED D/A CONVERTER

Signal Processing Technologies 

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 (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.

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 (ISet), the full-scale

output may be adjusted by varying the reference current. ISet

is controlled through the Ref+ input on the SPT1018. A

method and equations to set ISet is shown in figure 2. The

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 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

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 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.

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.

Figure 3 - Timing Diagram

CONV

-1.3 V

CONV

tPWH

tPWL

tS

tH

-1.3 V

Data Control

Inputs

tDST

tDSC

1/2 LSB

OUT -

OUT +

tSI

1/2 LSB

SPT

6

SPT1018

5/14/97

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