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SP9502

Dual, 12–Bit, Voltage Output D/A Converter

Signal Processing Technologies 

FEATURES

The SP9502 is a low power, dual version of the

popular SP9345, Quad 12-Bit Digital-to-Analog

Converter. This Dual, Voltage Output, 12-Bit

Digital-to-Analog Converter features ±4.5V output

swings when using ±5 volt supplies. The input

coding format used is standard offset binary.

(Refer to Table 1 below.)

The converter utilizes double-buffering on each of

the 12 parallel digital inputs, for easy microprocessor

interface. Each 12-bit DAC is independently

addressable and both DACS may be simulta-

neously updated using a single XFER command.

The output settling-time is specified at 4µs to full

12–bit accuracy when driving a 5Kohm, 50pf load

combination. The SP9502, Dual 12-Bit Digital-

to-Analog Converter is ideally suited for applica-

tions such as ATE, process controllers, robotics,

and instrumentation. The SP9502 is available in

28–pin plastic SOIC and plastic DIP packages,

specified over the commercial (0°C to +70°C)

temperature range.

THEORY OF OPERATION

The SP9502 consists of five main functional blocks

— input data multiplexer, data registers, control

logic,12-bit D/A converters, and two bipolar

output voltage amplifiers. The input data multi-

plexer is designed to interface to either 12- or 8-bit

microprocessor data busses. The input data format

is controlled by the B1/B2 signal — a logic “1”

selects the 12-bit mode, while a logic “0” selects

the 8-bit mode. In the 12-bit mode the data is

transferred to the input registers without changes

in its format. In the 8-bit mode, the four least

significant bits (LSBs) are connected to the four

INPUT

OUTPUT

MSB

LSB

1111 1111 1111

VREF - 1 LSB

1111 1111 1110

VREF - 2 LSB

1000 0000 0001

0 + 1 LSB

1000 0000 0000

0

0000 0000 0001

-VREF + 1 LSB

0000 0000 0000

-VREF

1 LSB = 2VREF

212

Table 1. Offset Binary Coding

most significant bits (MSBs), allowing an 8-bit

MSB-justified interface. All data inputs are

enabled using the CS signal in both modes. The

digital inputs are designed to be both TTL and 5V

CMOS compatible.

In order to reduce the DAC full scale output

sensitivity to the large weighting of the MSB’s

found in conventional R-2R resistor ladders, the 3

MSB’s are decoded into 8 equally weighted levels.

This reduces the contribution of each bit by a factor

of 4, thus, reducing the output sensitivity to mis-

matches in resistors and switches by the same

amount. Linearity errors and stability are both

improved for the same reasons. Each D/A con-

verter is separated from the data bus by two reg-

isters, each consisting of level-triggered latches,

Figure 1. The first register (input register) is 12-

bits wide. The input register is selected by the

address input A0 and A1 and is enabled by the CS

and WR1 signals. In the 8-bit mode, the enable

signal to the 8 MSB’s is disabled by a logic low

on B1/B2 to allow the 4 LSB’s to be updated. The

second register (DAC register), accepts the de-

coded 3 MSB’s plus the 9 LSB’s. The two DAC

registers are updated simultaneously for both

DAC’s using the XFER and WR2 signals. Using

the CLR and WR2 signals or the power-on-reset,

(enabled when the power is switched on) the DAC

registers are set to 1000 0000 0000 and the DAC

outputs will settle to 0V.

Using the control logic inputs, the user has full

control of address decoding, chip enable, data

transfer and clearing of the DAC’s. The control

logic inputs are level triggered, and like the data

inputs, are TTL and CMOS compatible. The truth

table (Table 2) shows the appropriate functions

associated with the states of the control logic

inputs.

The DACs themselves are implemented with a

precision thin–film resistor network and CMOS

transmission gate switches. Each D/A converter is

used to convert the 12-bit input from its DAC

register to a precision voltage.

The bipolar voltage output of the SP9502 is

created on-chip from the DAC Voltage Output

(V ) by using an operational amplifier and two

DAC

feedback resistors connected as shown in Figure 2.

This configuration produces a ±4.5V bipolar output

range with standard offset binary coding. (See Table 1)

SP9502DS/02

SP9502 Dual, 12-Bit, Voltage Output D/A Converter

5

© Copyright 1999 Sipex Corporation

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