www.clare.com
28
CPC5604
XXX
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.
Careful consideration should be taken related to PCB
layout of the snoop network. Traces should be as short
+0
-2.5
-5
-7.5
-10
-12.5
-15
-17. 5
-20
-22.5
-25
-27.5
-30
-32.5
-35
-37.5
-40
-42.5
-45
-47.5
-50
-52.5
-55
-57.5
-60
CMRR
(dBm)
5020 100 200 500 1K 2K 4K
Frequency (Hz)
Figure 10A Snoop Mode Common Mode Rejection Ratio Setup
RX-
RX+
TIP
RING
500 10H
500
10H
2.16uf
600
48V
+
-
40mA Loop Current
Audio Precision
System One
Analyzer
100K
V
IN1
IN2
AP1
LITELINK
TM
DAA Circuit
0dbm
Audio
Precision
Generator
RX
V
T/R
500uf
1%
matched
CID
Figure 10B Common Mode Rejection
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.
Figure 10A shows the test setup for CMRR through the
snoop signal path. For this test the LITELINK
TM
is on-
hook and the frequency is swept from 20Hz to 4kHz.
Figure 10B is a graph of CMRR vs. frequency.
Snoop Mode Common Mode Rejection Ratio (CMRR)
