DAC088S085_16 Datasheet, PDF (29/41 Pages) Texas Instruments – DAC088S085 8-Bit Micro Power OCTAL Digital-to-Analog Converter With Rail-to-Rail Outputs

www.ti.com
DAC088S085
SNAS424D – AUGUST 2007 – REVISED APRIL 2016
Using References as Power Supplies (continued)
An input capacitance of 1 µF without any ESR requirement is required at the LP3985 input, while a 1-µF ceramic
capacitor with an ESR requirement of 5 mΩ to 500 mΩ is required at the output. Careful interpretation and
understanding of the capacitor specification is required to ensure correct device operation.
10.1.4 LP2980
The LP2980 is an ultra-low dropout regulator with a ±0.5% or ±1% accuracy over temperature, depending upon
grade. It is available in 3 V, 3.3 V, and 5 V versions, among others.
Input
Voltage
V_IN
ON /OFF
LP2980
V_OUT
+
4.7 PF
VA VREF1,2
DAC088S085
0.1 PF
SYNC
DIN
SCLK
V_OUT = 0V
to 5V
Figure 46. Using the LP2980 Regulator
Like any low dropout regulator, the LP2980 requires an output capacitor for loop stability. This output capacitor
must be at least 1 µF over temperature, but values of 2.2 µF or more provides even better performance. The
ESR of this capacitor must be within the range specified in the LP2980 data sheet. Surface-mount solid tantalum
capacitors offer a good combination of small size and low ESR. Ceramic capacitors are attractive due to their
small size but generally have ESR values that are too low for use with the LP2980. Aluminum electrolytic
capacitors are typically not a good choice due to their large size and high ESR values at low temperatures.
11 Layout
11.1 Layout Guidelines
For best accuracy and minimum noise, the printed-circuit board containing the DAC088S085 must have separate
analog and digital areas. The areas are defined by the locations of the analog and digital power planes. Both of
these planes must be located in the same board layer. A single ground plane is preferred if digital return current
does not flow through the analog ground area. Frequently a single ground plane design uses a fencing technique
to prevent the mixing of analog and digital ground current. Separate ground planes must only be used when the
fencing technique is inadequate. The separate ground planes must be connected in one place, preferably near
the DAC088S085. Take special care to ensure that digital signals with fast edge rates do not pass over split
ground planes. They must always have a continuous return path below their traces.
For best performance, the DAC088S085 power supply must be bypassed with at least a 1-µF and a 0.1-µF
capacitor. The 0.1-µF capacitor must be placed right at the device supply pin. The 1-µF or larger valued
capacitor can be a tantalum capacitor while the 0.1-µF capacitor must be a ceramic capacitor with low ESL and
low ESR. If a ceramic capacitor with low ESL and low ESR is used for the 1-µF value and it can be placed right
at the supply pin, the 0.1-µF capacitor can be eliminated. Capacitors of this nature typically span the same
frequency spectrum as the 0.1-µF capacitor and thus eliminate the need for the extra capacitor. The power
supply for the DAC088S085 must only be used for analog circuits.
It is also advisable to avoid the crossover of analog and digital signals. This helps minimize the amount of noise
from the transitions of the digital signals from coupling onto the sensitive analog signals such as the reference
pins and the DAC outputs.
Copyright © 2007–2016, Texas Instruments Incorporated
Product Folder Links: DAC088S085
Submit Documentation Feedback
29