T7256 Agere Systems, T7256 Datasheet - Page 42

• Manufacturer Part Number: T7256
• Description: (T7234 - T7256) Compliance
• Manufacturer: Agere Systems

T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
Questions and Answers
S/T-Interface
A34: (continued)
38
b) Receiver Levels:
c) Resistor Selection:
T7250C: The line interface transformer has a
turns ratio of 2.5, and the transmitter drives a
line-side load of 50 . Reflecting this imped-
ance to the device side of the transformer
results in 312.5
tance, combined with the 113
tance of the device-side resistors, results in a
total of 425.5
drives. So, to optimize the transmitter part of
the circuit based on the load the transmitter
expects to drive, the transmitter should see a
total resistance of approximately 425 .
The T7234 S/T line interface transformer has
a turns ratio of 2.5. The receiver expects to
see nominal pulse levels of 750 mV x 2.5 =
1.875 V.
T7903: The transmitter circuit is a current
source of 7.5 mA. To generate a voltage of
1.875 V with 7.5 mA requires a resistance of
1.875/0.0075 = 250 .
T7250C: The transmitter circuit is a current
source of 6 mA. To generate a voltage of
1.875 V with 6 mA requires a resistance of
1.875/0.006 = 312.5 .
In this section, the term receiver implies not
only the receive section on the chip, but also
the external 10 k resistors connected to the
receiver. These resistors remain unchanged
from the standard line interface circuit in
order to maintain the same total receiver
impedance.
T7903: Ideally, the transmitter should be driv-
ing into 240 , and the T7234 receiver wants
to see the levels that would result if the trans-
mitter drove 7.5 mA through 250 . Since
these resistance values are so close, 249
is chosen as the resistor across which the
receiver is connected, and no other series
resistance is needed in the transmit path, as
Figure 20 illustrates.
T7250C: Ideally, the transmitter should be
driving into 425 , and the T7234 receiver
wants to see the levels that would result if the
(continued)
that the transmitter typically
(50
(continued)
x N
2
). This resis-
total resis-
CHIP PIN
Note that the receiver bias in Figure 16 (B) is
increased to 2.33 V (from 2.16 V in Figure 16
(A)). This is an increase of about 8% (0.67 dB).
This will decrease the overall receiver sensitivity
slightly. Normally, the receiver must have a sensi-
tivity to signals down to –7.5 dB of nominal.
Therefore, in the case of a direct connect, the
sensitivity is not an issue since the receiver will
always see a large input signal.
d) Receiver Bias:
100 k
transmitter drove 6 mA through 312.5 . So,
the total transmit path resistance should be
divided into three resistors. The first is the
resistor across which the receiver is con-
nected and should be approximately 312.5
so that the receiver sees the correct levels. A
standard 309
case. The remainder of the 425
divided equally between two other series
resistors in the transmit path, and (425
309 )/2 is 58.0 , so a standard 57.6
value is chosen for the two other series resis-
tors as illustrated in Figure 21.
Normally, the transmitter of theT7903/
T7250C is biased at 5 V through 100 k pull-
up, and the receiver of the T7234 is biased at
2.16 V through a resistor network that can be
simplified as shown in Figure 16 (A). When
the direct-connect scheme is implemented,
the resulting network between the T7903/
T7250C transmitter and the T7234 receiver is
as shown in Figure 16 (B).
(A)
5 V
Figure 16. Receiver Bias
30 k
2.16 V
23 k
value is adequate for this
5 V
CHIP PIN
100 k
Lucent Technologies Inc.
10 k
February 1998
CHIP PIN
100 k
should be
5 V
(B)
30 k
2.33 V
23 k
5-4726
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