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
N
2
a
1
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
and
S
e
0
POWER OUT
POWER IN
0
Z2
Z1
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
J
R3
e
Z1
Note
See Application Note 370 for further details
N
N
2
b
1
2
b
2NS
a
1
12
NSC [ NATIONAL SEMICONDUCTOR ]
