7

®

DAC811

DAC811 features separate digital and analog power supply

returns to permit optimum connections for low noise and

high speed performance. The analog common (pin 23) and

digital common (pin 15) should be connected together at one

point. Separate returns minimize current flow in low level

signal paths if properly connected. Logic return currents are

not added into the analog signal return path. A

±0.5V

difference between ACOM and DCOM is permitted for

specified operation. High frequency noise on DCOM with

respect to ACOM may permit noise to be coupled through to

the analog output; therefore, some caution is required in

applying these common connections.

The Analog Common is the high quality return for the D/A

converter and should be connected directly to the analog

reference point of the system. The load driven by the output

amplifier should be returned to the Analog Common.

EXTERNAL OFFSET AND GAIN ADJUSTMENT

Offset and Gain may be trimmed by installing external

Offset and Gain potentiometers. Connect these potentiom-

eters as shown in Figure 5. TCR of the potentiometers

should be 100ppm/

°C or less. The 1MΩ and 3.9MΩ resis-

tors (20% carbon or better) should be located close to the

DAC811 to prevent noise pickup. If it is not convenient to

use these high value resistors, an equivalent “T” network, as

shown in Figure 6, may be substituted in each case. The

Gain Adjust (pin 22) is a high impedance point and a

0.001

µF to 0.01µF ceramic capacitor should be connected

from this pin to Analog Common to reduce noise pickup in

all applications, including those not employing external gain

adjustment. Excessive capacitance on the Gain Adjust or

Offset Adjust pin may affect slew rate and settling time.

FIGURE 6. Equivalent Resistances.

FIGURE 7. Output Amplifier Voltage Range Scaling Circuit.

OUTPUT

DIGITAL

CONNECT

CONNECT

RANGE

INPUT CODES

PIN 25 TO

PIN 27 TO

0 to +10V

USB

24

23

±5

BOB or BTC

24

26

±10V

BOB or BTC

NC

26

TABLE IV. Output Range Connections.

APPLICATIONS

MICROCOMPUTER BUS INTERFACING

The DAC811 interface logic allows easy interface to micro-

computer bus structures. The control signal WR is derived

from external device select logic and the I/O Write or

Memory Write (depending upon the system design) signals

from the microcomputer.

which of the latches are enabled. It is permissible to enable

two or more latches simultaneously, as shown in some of the

following examples.

The double-buffered latch permits data to be loaded into the

input latches of several DAC811s and later strobed into the

D/A latch of all D/As, simultaneously updating all analog

outputs. All the interface schemes shown below use a base

address decoder. If blocks of memory are used, the base

address decoder can be simplified or eliminated altogether.

For instance, if half the memory space is unused, address

line A15 of the microcomputer can be used as the chip select

control.

4-BIT INTERFACE

An interface to a 4-bit microcomputer is shown in Figure 8.

Each DAC811 occupies four address locations. A 74LS139

provides the two-to-four decoder and selects it with the base

address. Memory Write (WR) of the microcomputer is

connected directly to the WR pin of the DAC811. An 8205

decoder is an alternative to the 74LS139.

OUTPUT RANGE CONNECTIONS

Internal scaling resistors provided in the DAC811 may be

connected to produce bipolar output voltage ranges of

±10V

and

±5V or a unipolar output voltage range of 0 to +10V.

The 20V range (

±10V bipolar range) is internally connected.

Refer to Figure 7. Connections for the output ranges are

listed in Table IV.

1M

Ω

3.9M

Ω

100k

Ω

100k

Ω

12k

Ω

10k

Ω

180k

Ω

180k

Ω

4.26k

Ω

5.36k

Ω

24

V

OUT

23

Analog Common

From D/A

Converter

From Voltage

Reference

4.26k

Ω

25

10V Range

26

Summing Junction

27

Bipolar Offset