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參數(shù)資料
| 型號： | MC33121 |
| 廠商： | Motorola, Inc. |
| 英文描述： | LOW VOLTAGE SUBSCRIBER LOOP INTERFACE CIRCUIT |
| 中文描述： | 低電壓用戶環(huán)路接口電路 |
| 文件頁數(shù)： | 25/32頁 |
| 文件大?。?/td> | 570K |
| 代理商： | MC33121 |
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MC33121
MOTOROLA
25
Basically, digital connections to ST1 and ST2 should be
referenced to the VDD and VDG pins, while the transmit and
receive analog signals should be referenced to the analog
ground (VAG). VCC should be connected to a clean battery
ground, and generally should not be connected directly to
VDG and/or VAG (on the line card) when strong transients are
anticipated. Even with a good layout, VCC can move several
volts when a transient hits, possibly damaging components
on the + 5.0 V line if their grounds have a direct connection at
the line card. The MC33121 is designed to allow VCC to
move as much as
±
30 V with respect to VDG and VAG on a
transient basis only. VCC and the other grounds should pre-
ferably be connected together at the power supply rather
than at the IC. Internally, the MC33121 has clamp diodes on
the 4–wire side pins as indicated in Figure 40.
If the codec has a single ground pin, as in Figure 41, it will
be the reference for both the digital and analog signals, and
must be connected to both VAG and VDG on the MC33121. If
the codec has separate digital and analog grounds, as in Fig-
ure 42 (the MC145503 internally generates the analog
ground), then each ground should be connected to the
appropriate ground on the MC33121.
e) Other
A 0.1
μ
F capacitor should be provided across VCC to VEE
on the MC33121 to help keep idle channel noise to a mini-
mum.
The CQB capacitor (on the VQB pin) forms a pole with an
internal 7.5 k
resistor to filter noise from the VEE pin, pro-
viding an internal quiet battery supply for the speech ampli-
fiers. Power supply rejection will depend on the value and
quality of this capacitor at the frequencies of concern. Tanta-
lum capacitors generally have better high frequency charac-
teristics then electrolytics. See Figure 17 and 18 for ripple
rejection characteristics (the four–wire data was measured at
pin 11 (TXO)). Figure 16 indicates ripple rejection from the
+ 5.0 V supply (VDD).
In general, pc board tracks carrying analog signals (on the
four–wire side and Tip/Ring) should not be routed through
the digital section where they could pick up digital noise. Any
tracks longer than a few inches should be considered an an-
tenna and should be checked for potential noise or RFI pick-
up which could affect the circuit operation.
Figure 41. Connection to a CODEC With a Single Ground
–
+
–
+
VCC
VDD
MC33121
+5.0 V
VCC
VDG
VAG
RXI
TXO
VEE
VQB
GNDA
VFRO
VFXI+
VFXI–
GSX
VBB
–5.0 V
MC145554
BATTERY GROUND
DIGITAL GROUND
BATTERY SUPPLY
10
μ
F
0.1
0.1
5
μ
F
0.1
0.1
DIGITAL
INTERFACE
Figure 42. Connection to a CODEC With Separate Grounds
–
+
–
+
VCC
VDD
MC33121
+5.0 V
VCC
VDG
VAG
RXI
TXO
VEE
VQB
VLS
VAG
RXO
–TX
TXI
VSS
–5.0 V
MC145503
BATTERY GROUND
DIGITAL GROUND
BATTERY SUPPLY
DIGITAL
INTERFACE
10
μ
F
0.1
0.1
5
μ
F
0.1
0.1
+TX
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