Datasheet

DAC8534
18
SBAS254D
www.ti.com
The output voltage for any input code can be calculated as
follows:
VXV
D
RR
R
V
R
R
OUT
REF REF
=•






•
+






•














65536
12
1
2
1
–
where D represents the input code in decimal (0–65535).
With V
REF
= 5V, R
1
= R
2
= 10kΩ:
VX
D
V
OUT
=
•






10
65536
5–
This is an output voltage range of ±5V with 0000
H
corre-
sponding to a –5V output and FFFF
H
corresponding to a +5V
output. Similarly, using V
REF
= 2.5V, a ±2.5V output voltage
range can be achieved.
LAYOUT
A precision analog component requires careful layout, ad-
equate bypassing, and clean, well-regulated power supplies.
The DAC8534 offers single-supply operation, and 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 DAC8534, all return
currents, including digital and analog return currents for the
DAC, must flow through a single point. Ideally, GND would
be connected directly to an analog ground plane. This plane
would be separate from the ground connection for the digital
components until they were connected at the power-entry
point of the system.
The power applied to AV
DD
should be well regulated and low
noise. Switching power supplies and DC/DC converters will
often have high-frequency glitches or spikes riding on the
output voltage. In addition, digital components can create
similar high-frequency spikes as their internal logic switches
states. This noise can easily couple into the DAC output
voltage through various paths between the power connec-
tions and analog output.
As with the GND connection, AV
DD
should be connected to
a positive power-supply plane or trace that is separate from
the connection for digital logic until they are connected at the
power-entry point. In addition, a 1µF to 10µF capacitor in
parallel with a 0.1µF bypass capacitor is strongly recom-
mended. In some situations, additional bypassing may be
required, such as a 100µF electrolytic capacitor or even a
“Pi” filter made up of inductors and capacitors—all designed
to essentially low-pass filter the supply, removing the high-
frequency noise.
Up to four DAC8534 devices can be used on a single SPI bus
without any glue logic to create a high channel count solu-
tion. Special attention is required to avoid digital signal
integrity problems when using multiple DAC8534s on the
same SPI bus. Signal integrity of
SYNC , SCLK, and D
IN
lines
will not be an issue as long as the rise times of these digital
signals are longer than six times the propagation delay
between any two DAC8534 devices. Propagation speed is
approximately six inches/ns on standard PCBs. Therefore, if
the digital signal risetime is 1ns, the distance between any
two DAC8534 devices is recommended not to exceed 1 inch.
If the DAC8534s have to be further apart on the PCB, the
signal rise times should be reduced by placing series resis-
tors at the drivers for
SYNC , SCLK, and D
IN
lines. If the
largest distance between any two DAC8534s has to be six
inches, the risetime should be reduced to 6ns with an RC
network formed by the series resistor at the digital driver and
the total trace and input capacitance on the PCB.