DAC712
SBAS023A–SEPTEMBER 2000–REVISED JULY 2009 ................................................................................................................................................. www.ti.com
Gain Adjustment
1
2
DCOM
ACOM
VOUT
28
27
26
25
24
23
22
21
20
19
18
17
16
15
Apply the digital input that gives the maximum
positive voltage output. Adjust the gain potentiometer
or the gain adjust D/A converter for this positive
full-scale voltage.
3
4
INSTALLATION
5
VREF OUT
6
GENERAL CONSIDERATIONS
+12V to +15V
7
+VCC
Because of the high accuracy of these D/A
converters, system design problems such as
grounding and contact resistance become very
important. A 16-bit converter with a 20V full-scale
range has a 1LSB value of 305mV. With a load
current of 5µA, series wiring and connector
resistance of only 60mΩ causes a voltage drop of
300µV. To understand what this means in terms of a
system layout, the resistivity of a typical 1-ounce
copper-clad printed circuit board (PCB) is 1/2mΩ per
square. For a 5mA load, a 10 mil (0.010 inch) wide
printed circuit conductor 60 milli-inches long results in
a voltage drop of 150µV.
-12V to -15V
8
-VCC
+
0.01mF
9
10
11
12
13
14
0.01mF
+
Figure 11. Power-Supply Connections
The analog output of the DAC712 has an LSB size of
305µV (–96dB). The noise floor of the D/A converter
must remain below this level in the frequency range
of interest. The DAC712 noise spectral density (which
includes the noise contributed by the internal
reference) is shown in the Typical Characteristics
section.
The DAC712 has separate ANALOG COMMON and
DIGITAL COMMON pins. The current through DCOM
is mostly switching transients and are up to 1mA
peak in amplitude. The current through ACOM is
typically 5µA for all codes.
Use separate analog and digital ground planes with a
single interconnection point to minimize ground loops.
The analog pins are located adjacent to each other to
help isolate analog from digital signals. Analog
signals should be routed as far as possible from
digital signals and should cross them at right angles.
Wiring to high-resolution D/A converters should be
routed to provide optimum isolation from sources of
radio
frequency
interference
(RFI)
and
electromagnetic interference (EMI). The key to
elimination of RF radiation or pickup is a small loop
area. Signal leads and the return conductors should
be kept close together such that they present a small
capture cross-section for any external field.
Wire-wrap construction is not recommended.
A
solid analog ground plane around the D/A
converter package, as well as under it in the vicinity
of the analog and power-supply pins, isolates the D/A
converter from switching currents. It is recommended
that DCOM and ACOM be connected directly to the
ground planes under the package.
POWER-SUPPLY AND REFERENCE
CONNECTIONS
If several DAC712s are used, or if the DAC712
shares supplies with other components, connecting
the ACOM and DCOM lines together once at the
power supplies rather than at each chip may give
better results.
Power-supply decoupling capacitors should be added
as shown in Figure 11. Best performance occurs
using a 1µF to 10µF tantalum capacitor at –VCC
.
Applications with less critical settling time may be
able to use 0.01µF at –VCC as well as at +VCC. The
capacitors should be located close to the package.
14
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