Datasheet

500 Ω
I/VSum
Amp
Cext
Rint/Rext
Rint/Rext
CW_OUTM
CW_OUTP
LNA
INP
INM
ACT
INPUT
CW_AMPINM
CW_AMPINP
Mixer
Clock
PGA_P
PGA_M
Cext
5 K
5K
500 Ω
S0504-01
AFE5808
SLOS688C –SEPTEMBER 2010–REVISED APRIL 2012 www.ti.com
Figure 91. AFE5808 PGA Test Mode
POWER SUPPLY, GROUNDING AND BYPASSING
In a mixed-signal system design, power supply and grounding design plays a significant role. The AFE5808
distinguishes between two different grounds: AVSS(Analog Ground) and DVSS(digital ground). In most cases, it
should be adequate to lay out the printed circuit board (PCB) to use a single ground plane for the AFE5808.
Care should be taken that this ground plane is properly partitioned between various sections within the system to
minimize interactions between analog and digital circuitry. Alternatively, the digital (DVDD) supply set consisting
of the DVDD and DVSS pins can be placed on separate power and ground planes. For this configuration, the
AVSS and DVSS grounds should be tied together at the power connector in a star layout. In addition, optical
isolator or digital isolators, such as ISO7240, can separate the analog portion from the digital portion completely.
Consequently they prevent digital noise to contaminate the analog portion. Table 13 lists the related circuit blocks
for each power supply.
Table 14. Supply vs Circuit Blocks
Power Supply Ground Circuit Blocks
LNA, attenuator, PGA with clamp
and BPF, reference circuits, CW
AVDD (3.3VA) AVSS
summing amplifier, CW mixer,
VCA SPI
LNA, CW clock circuits, reference
AVDD_5V (5VA) AVSS
circuits
ADC analog and reference
AVDD_ADC (1.8VA) AVSS
circuits
DVDD (1.8VD) DVSS LVDS and ADC SPI
All bypassing and power supplies for the AFE5808 should be referenced to their corresponding ground planes.
All supply pins should be bypassed with 0.1µF ceramic chip capacitors (size 0603 or smaller). In order to
minimize the lead and trace inductance, the capacitors should be located as close to the supply pins as possible.
Where double-sided component mounting is allowed, these capacitors are best placed directly under the
package. In addition, larger bipolar decoupling capacitors 2.2µF to 10µF, effective at lower frequencies) may also
be used on the main supply pins. These components can be placed on the PCB in proximity (< 0.5 in or 12.7
mm) to the AFE5808 itself.
The AFE5808 has a number of reference supplies needed to be bypassed, such CM_BYP, VHIGH, and
VREF_IN. These pins should be bypassed with at least 1µF; higher value capacitors can be used for better low-
frequency noise suppression. For best results, choose low-inductance ceramic chip capacitors (size 0402, > 1µF)
and place them as close as possible to the device pins.
62 Submit Documentation Feedback Copyright © 2010–2012, Texas Instruments Incorporated
Product Folder Links: AFE5808