Datasheet

Applications Information
POWER SUPPLIES
While the pins of the TP305X family are well protected
against electrical misuse, it is recommended that the stan-
dard CMOS practice be followed, ensuring that ground is
connected to the device before any other connections are
made. In applications where the printed circuit board may be
plugged into a ‘‘hot’’ socket with power and clocks already
present, an extra long ground pin in the connector should
be used.
All ground connections to each device should meet at a
common point as close as possible to the GNDA pin. This
minimizes the interaction of ground return currents flowing
through a common bus impedance. 0.1 mF supply decou-
pling capacitors should be connected from this common
ground point to V
CC
and V
BB
, as close to the device as
possible.
For best performance, the ground point of each CODEC/
FILTER on a card should be connected to a common card
ground in star formation, rather than via a ground bus.
This common ground point should be decoupled to V
CC
and
V
BB
with 10 mF capacitors.
RECEIVE GAIN ADJUSTMENT
For applications where a TP305X family CODEC/filter re-
ceive output must drive a 600X load, but a peak swing lower
than
g
2.5V is required, the receive gain can be easily ad-
justed by inserting a matched T-pad or
q-pad at the output.
Table II lists the required resistor values for 600X termina-
tions. As these are generally non-standard values, the equa-
tions can be used to compute the attenuation of the closest
practical set of resistors. It may be necessary to use un-
equal values for the R1 or R4 arms of the attenuators to
achieve a precise attenuation. Generally it is tolerable to
allow a small deviation of the input impedance from nominal
while still maintaining a good return loss. For example a 30
dB return loss against 600X is obtained if the output imped-
ance of the attenuator is in the range 282X to 319X (as-
suming a perfect transformer).
T-Pad Attenuator
R1
e
Z1
#
N
2
a
1
N
2
b
1
J
b
2
0
Z1.Z2
#
N
N
2
b
1
J
R2
e
2
0
Z1.Z2
#
N
N
2
b
1
J
Where: N
e
0
POWER IN
POWER OUT
and
S
e
0
Z1
Z2
Also: Z
e
0
Z
SC
#
Z
OC
Where Z
SC
e
impedance with short circuit termination
and Z
OC
e
impedance with open circuit termination
q-Pad Attenuator
TL/H/5510– 5
R3
e
0
Z1.Z2
2
#
N
2
b
1
N
J
R3
e
Z1
#
N
2
b
1
N
2
b
2NS
a
1
J
Note: See Application Note 370 for further details.
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