Datasheet

8
®
DAC715
DAC715 CALIBRATION VALUES
1 LEAST SIGNIFICANT BIT = 152
µV
DIGITAL INPUT CODE ANALOG
BINARY TWO’S OUTPUT
COMPLEMENT, BTC (V) DESCRIPTION
7FFF
H
9.999847 Full Scale –1LSB
|
4000
H
7.5 3/4 Scale
|
0001
H
5.000152 Half Scale + 1LSB
0000
H
5 Half Scale
FFFF
H
4.999847 Half Scale – 1LSB
|
C000
H
2.5 1/4 Scale
|
8000
H
0 Zero
Gain Adjustment
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.
INSTALLATION
GENERAL CONSIDERATIONS
Due to the high-accuracy of the DAC715, system design
problems such as grounding and contact resistance become
very important. A 16-bit converter with a 10V full-scale
range has a 1LSB value of 152µV. With a load current of
5mA, series wiring and connector resistance of only 60mΩ
will cause 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 is 1/2 mΩ per
square. For a 5mA load, a 10 milliinch wide printed circuit
conductor 60 milliinches long will result in a voltage drop of
150µV.
The analog output of DAC715 has an LSB size of 152µV
(–96dB). The noise floor of the D/A must remain below this
level in the frequency range of interest. The DAC715’s noise
spectral density (which includes the noise contributed by the
internal reference) is shown in the Typical Performance
Curves section.
Wiring to high-resolution D/A converters should be routed
to provide optimum isolation from sources of RFI and EMI.
The key to elimination of RF radiation or pickup is small
loop area. Signal leads and their 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.
POWER SUPPLY AND
REFERENCE CONNECTIONS
Power supply decoupling capacitors should be added as
shown in Figure 4. Best performance occurs using a 1 to
10µF tantalum capacitor at –V
CC
. Applications with less
critical settling time may be able to use 0.01µF at –V
CC
as
TABLE I. Digital Input and Analog Output Voltage Calibra-
tion Values.
well as at +V
CC
. The capacitors should be located close to the
package.
The DAC715 has separate ANALOG COMMON and DIGI-
TAL 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. A solid analog ground plane around the D/A
package, as well as under it in the vicinity of the analog and
power supply pins, will isolate the D/A from switching
currents. It is recommended that DCOM and ACOM be
connected directly to the ground planes under the package.
If several DAC715s are used or if DAC715 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.
LOAD CONNECTIONS
Since the reference point for V
OUT
and V
REF OUT
is the ACOM
pin, it is important to connect the D/A converter load
directly to the ACOM pin. Refer to Figure 5.
Lead and contact resistances are represented by R
1
through
R
3
. As long as the load resistance R
L
is constant, R
1
simply
introduces a gain error and can be removed by gain adjust-
ment of the D/A or system-wide gain calibration. R
2
is part
of R
L
if the output voltage is sensed at ACOM.
In some applications it is impractical to return the load to the
ACOM pin of the D/A converter. Sensing the output voltage
at the SYSTEM GROUND point is reasonable, because
there is no change in DAC715 ACOM current, provided that
FIGURE 4. Power Supply Connections.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
+
0.01µF
DCOM
ACOM
V
OUT
V
REF OUT
+V
CC
–V
CC
0.01µF
+
+12V to +15V
–12V to –15V