ADS7251 Datasheet, PDF (28/38 Pages) Texas Instruments – ADS7x51 12-Bit, 2-MSPS and 14-Bit, 1.5-MSPS, Dual, Differential Input, Simultaneous- Sampling, Analog-to-Digital Converters with Internal Reference

ADS7251, ADS7851
SBAS587A – JANUARY 2014 – REVISED APRIL 2014
10 Layout
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10.1 Layout Guidelines
Figure 52 shows a board layout example for the ADS7251 and ADS7851. Use a ground plane underneath the
device and partition the PCB into analog and digital sections. Avoid crossing digital lines with the analog signal
path and keep the analog input signals and the reference input signals away from noise sources. As shown in
Figure 52, the analog input and reference signals are routed on the left side of the board and the digital
connections are routed on the right side of the device.
The power sources to the device must be clean and well-bypassed. Use 10-μF, ceramic bypass capacitors in
close proximity to the analog (AVDD) and digital (DVDD) power-supply pins. Avoid placing vias between the
AVDD and DVDD pins and the bypass capacitors. Connect all ground pins to the ground plane using short, low-
impedance paths.
The REFOUT-A and REFOUT-B reference outputs are bypassed with 10-μF, X7R-grade ceramic capacitors
(C_REF-x). Place the reference bypass capacitors as close as possible to the reference REFOUT-x pins and
connect the bypass capacitors using short, low-inductance connections. Avoid placing vias between the
REFOUT-x pins and the bypass capacitors. Small 0.1-Ω to 0.2-Ω resistors (R_REF-x) are used in series with the
reference bypass capacitors to improve stability.
The fly-wheel RC filters are placed immediately next to the input pins. Among ceramic surface-mount capacitors,
COG (NPO) ceramic capacitors provide the best capacitance precision. The type of dielectric used in COG
(NPO) ceramic capacitors provides the most stable electrical properties over voltage, frequency, and temperature
changes. Figure 52 shows C_IN-A and C_IN-B filter capacitors placed across the analog input pins of the device.
10.2 Layout Example
C_REF-A
GND
C_IN-A
AVDD
GND
C_AVDD
GND
GND
REFOUT-A
REFGND-A
REFGND-B
REFOUT-B
GND
SDO-A
SDO-B
SCLK
/CS
GND
C_REF-B
C_IN-B
C_DVDD
DVDD
GND
GND
Figure 52. Example Layout for the ADS7251 and ADS7851
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