CPC5604
Snoop Mode Common Mode Rejection Ratio (CMRR)
as possible and kept equidistant from one another.
Spacing of 0.1” should be maintained between traces
on the phone line side. If possible, traces should be
routed away from large 60Hz fields to prevent noise
inducement into the snoop circuit.
As a practical matter, CMRR is dependent on how well
the external snoop network CS and RSNOOP are
matched. It is recommended that capacitors CS (C6 or
C7) be ceramic NPO (COG) type for excellent tempera-
ture stability and have a tolerance of 5% or less.
Resistor tolerance for RSNOOP (R5 or R6) should also
be at least 5% or better.
Figure 10A shows the test setup for CMRR through the
snoop signal path. For this test the LITELINKTM is on-
hook and the frequency is swept from 20Hz to 4kHz.
Figure 10B is a graph of CMRR vs. frequency.
Careful consideration should be taken related to PCB
layout of the snoop network. Traces should be as short
Figure 10A Snoop Mode Common Mode Rejection Ratio Setup
40mA Loop Current
TIP
V
RX+
IN1
Audio Precision
System One
Analyzer
10H
500
V
AP1
IN2
100K
RX
+
-
48V
RX-
LITELINKTM DAA Circuit
T/R
2.16uf
RING
CID
500
10H
500uf
1%
matched
600
Audio
Precision
Generator
0dbm
Figure 10B Common Mode Rejection
+0
-2.5
-5
-7.5
-10
-12.5
-15
-17.5
-20
-22.5
-25
-27.5
CMRR
(dBm)
-30
-32.5
-35
-37.5
-40
-42.5
-45
-47.5
-50
-52.5
-55
-57.5
-60
20
50
100
200
500
1K
2K
4K
Frequency (Hz)
XXX
28
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