AD8123ACPZ Analog Devices Inc, AD8123ACPZ Datasheet - Page 14

• Manufacturer Part Number: AD8123ACPZ
• Description: Triple Equalizer
• Manufacturer: Analog Devices Inc

AD8123
POWER SUPPLY FILTERING
External power supply filtering between the system power
supplies and the AD8123 is required in most applications to
prevent supply noise from contaminating the received signal as
well as to prevent unwanted feedback through the supplies that
could cause instability. Figure 29 shows that the AD8123 power
supply rejection decreases with increasing frequency. These
plots are for the lowest control settings and shift upward as the
peaking is increased.
A suitable filter that uses a surface-mount ferrite bead is shown
in Figure 30, and its frequency response is shown in Figure 31.
Because the frequency response was taken using a 50 Ω network
analyzer and with only one 0.1 μF capacitor on the AD8123
side, the actual amount of rejection provided by the filter in a
real-world application will be different from that shown in
Figure 31. The general shape of the rejection curve, however,
matches Figure 31, providing substantially increased overall
PSRR from approximately 5 MHz to 500 MHz, where it is most
needed. One filter is required on each of the two supplies (not one
filter per supply pin).
SYSTEM
*ALL AD8123 SUPPLY PINS ARE INDIVIDUALLY
SUPPLY
–10
–20
–30
–40
–50
–60
DECOUPLED WITH A 0.1µF CAPACITOR.
10
0
100k
V
V
V
GAIN
PEAK
POLE
Figure 29. AD8123 PSRR vs. Frequency
= 0V
= 0V
= 0V
Figure 30. Power Supply Filter
0.1µF
1M
FREQUENCY (Hz)
4700pF
2743021447
FAIR-RITE
10M
4700pF
TO AD8123*
100M
+PSRR
–PSRR
Rev. A | Page 14 of 16
LAYOUT AND POWER SUPPLY DECOUPLING
CONSIDERATIONS
Standard high speed PCB layout practices should be adhered
to when designing with the AD8123. A solid ground plane is
required and controlled impedance traces should be used when
interconnecting the high speed signals. Source termination
resistors on all of the outputs must be placed as close as possible
to the output pins.
The exposed paddle on the underside of the AD8123 must be
connected to a pad that connects to at least one PCB plane.
Several thermal vias should be used to make the connection
between the pad and the plane(s).
High quality 0.1 μF power supply decoupling capacitors should
be placed as close as possible to all of the supply pins. Small
surface-mount ceramic capacitors should be used for these, and
tantalum capacitors are recommended for bulk supply decoupling.
Figure 31. Power Supply Filter Frequency Response in a 50 Ω System
–100
–120
–20
–40
–60
–80
0
10k
100k
FREQUENCY (Hz)
1M
10M
100M