DAC707SH datasheet(14/27 Pages) TI

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DAC8531

SBAS192B

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power supply is quite noisy or if the system supply voltages

are at some value other than 5V. The REF02 will output a

steady supply voltage for the DAC8531. If the REF02 is

used, the typical current it needs to supply to the DAC8531

is 250

µA. This is with no load on the output of the DAC.

When the DAC output is loaded, the REF02 also needs to

supply the current to the load. The total current required (with

a 5k

Ω load on the DAC output) is:

250

µA + (5V/5kΩ) = 1.29mA

The load regulation of the REF02 is typically 0.005%/mA,

which results in an error of 322

µV for the 1.29mA current

drawn from it. This corresponds to a 4.2LSB error.

BIPOLAR OPERATION USING THE DAC8531

The DAC8531 has been designed for single-supply operation

but a bipolar output range is also possible using the circuit in

Figure 10. The circuit shown will give an output voltage range

±V

REF. Rail-to-rail operation at the amplifier output is achiev-

able using an OPA703 as the output amplifier.

The output voltage for any input code can be calculated as

follows:

REF

=• 







•









•





















65536

–

where D represents the input code in decimal (0–65535).

With VREF = 5V, R1 = R2 = 10kΩ:

O =

•









65536

–

This is an output voltage range of

±5V with 0000

H corre-

sponding to a –5V output and FFFFH corresponding to a +5V

output. Similarly, using VREF = 2.5V, ±2.5V output voltage

raw can be achieved.

LAYOUT

A precision analog component requires careful layout, ad-

equate bypassing, and clean, well-regulated power supplies.

As the DAC8531 offers single-supply operation, it will often

be used in close proximity with digital logic, microcontrollers,

microprocessors, and digital signal processors. The more

digital logic present in the design and the higher the switch-

ing speed, the more difficult it will be to keep digital noise

from appearing at the output.

Due to the single ground pin of the DAC8531, all return

currents, including digital and analog return currents, must

flow through the GND pin. Ideally, GND would be connected

directly to an analog ground plane. This plane would be

separate from the ground connection for the digital compo-

nents until they were connected at the power-entry point of

the system.

The 68HC11 should be configured so that its CPOL bit is a

0 and its CPHA bit is a 1. This configuration causes data

appearing on the MOSI output to be valid on the falling edge

of SCK. When data is being transmitted to the DAC, the

SYNC line is taken LOW (PC7). Serial data from the 68HC11

is transmitted in 8-bit bytes with only eight falling clock edges

occurring in the transmit cycle. Data is transmitted MSB first.

In order to load data to the DAC8531, PC7 is left LOW after

the first eight bits are transferred, then a second and third

serial write operation is performed to the DAC and PC7 is

taken HIGH at the end of this procedure.

APPLICATIONS

USING REF02 AS A POWER SUPPLY FOR

THE DAC8531

Due to the extremely low supply current required by the

DAC8531, an alternative option is to use a REF02 +5V

precision voltage reference to supply the required voltage to

the part, as shown in Figure 9. This is especially useful if the

FIGURE 8. DAC8531 to 68HC11 Interface.

FIGURE 9. REF02 as a Power Supply to the DAC8531.

DAC8531 TO 68HC11 INTERFACE

Figure 8 shows a serial interface between the DAC8531 and

the 68HC11 microcontroller. SCK of the 68HC11 drives the

SCLK of the DAC8531, while the MOSI output drives the

serial data line of the DAC. The SYNC signal is derived from

a port line (PC7), similar to what was done for the 8051.

68HC11(1)

PC7

SCK

MOSI

SYNC

SCLK

DAC8531(1)

NOTE: (1) Additional pins omitted for clarity.

REF02

DAC8531

Three-Wire

Serial

Interface

+5V

285

µA

OUT = 0V to 5V

SYNC

SCLK

+15

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