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
0.001µF 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.
From Voltage
Reference
5.36kΩ
27
26
4.26kΩ
25
10V Range
Bipolar Offset
Summing Junction
From D/A
Converter
4.26kΩ
24
V
OUT
23
Analog Common
FIGURE 7. Output Amplifier Voltage Range Scaling Circuit.
OUTPUT
RANGE
0 to +10V
±5
±10V
DIGITAL
INPUT CODES
USB
BOB or BTC
BOB or BTC
CONNECT
PIN 25 TO
24
24
NC
CONNECT
PIN 27 TO
23
26
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.
The latch enable lines N
A
, N
B
, N
C
and LDAC determine
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.
1MΩ
100kΩ
100kΩ
12kΩ
3.9MΩ
180kΩ
180kΩ
10kΩ
FIGURE 6. Equivalent Resistances.
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.
®
7
DAC811
ETC [ ETC ]
