CH1837A Datasheet, PDF (2/9 Pages) List of Unclassifed Manufacturers – CH1837A/7F/8A Data Access Arrangement Module V.34bis High Speed DAA Module

Cermetek Microelectronics, Inc.
Ring Detection. To announce an incoming call, the
telephone company’s central office (CO) applies an
AC ringing signal to the PSTN line. The
CH1837A/7F/8A is designed to detect this signal. The
CH1837A/7F/8A uses the AC ring signal to produce a
square wave pulse train output at the CH1837A/7F/8A
RI pin. This pulse train is the same frequency as the
incoming AC ring signal and is typically 20 Hz. Figure
3 shows external filtering which may be employed to
provide an envelope indication of the ring signal using
the CH1837A/7F/8A RI output. The ring detection
circuit in the CH1837A/7F/8A family is designed to
deter false indications caused by pulse dialing or noise
on the PSTN line.
Additionally, the RI output of the CH1837A/7F/8A is
diode protected thereby allowing an external pull-up
resistor (R>100k ohms) to +5V to be utilized to
activate the ring detection circuit when the
CH1837A/7F/8A is not connected to power. This can
be handy for designs in which power consumption is of
concern. When circuited in this manner, there is
virtually no current draw until a ring signal is present.
Refer to Figure 4.
CH1837A/7F/8A Data Access Arrangement Module
When the OFFHK input is TTL High, the
CH1837A/7F/8A is Off-Hook. The Off-Hook state
indicates to the CO that the CH1837A/7F/8A is either
answering a call or preparing to place a call.
OFFHK Input:
Active HIGH
OFFHK active Current: 5mA
RI Output:
Sensitivity:
Opt coupler, 30KΩ pull-up, Active
LOW
Square wave 15-68HZ (TIP. 20Hz)
38Vrms across TIP and RING
Figure 3. Typical Application.
Hook Switch Control.
The OFFHK input is used to toggle the
CH1837A/7F/8A from On-Hook to Off-Hook and back
to On-Hook.
When the OFFHK input is TTL Low, the
CH1837A/7F/8A is On-Hook. The On-Hook state
indicates to the CO that the CH1837A/7F/8A is ready
to receive calls.
Figure 4. Low Power Ring Detection Circuit.
2-Wire to 4-Wire Hybrid Converter (CH1837A/F
only). This circuit block performs two functions:
1. It applies the XMIT signal to the PSTN line, and
2. It subtracts the XMIT signal from the total signal
present on the PSTN line thereby deriving the
RCV signal.
The accuracy of the RCV signal derivation depends on
how closely the impedance of the PSTN line matches
an ideal 600 Ohms. Generally, a small amount of the
XMIT signal will appear at RCV due to the non-ideal
nature of the PSTN line impedance. The ratio of the
applied XMIT signal to the received XMIT signal is
called Trans-Hybrid Rejection, or Trans-Hybrid Loss.
PSTN Line Isolation and Surge/Peak Voltage
Protection. The Public Switched Telephone Network
(PSTN) line isolation barrier circuitry provides isolation
and the required surge and high voltage protection.
This circuitry effectively isolates the user’s equipment
from both the PSTN line and the CO. The
CH1837A/7F/8A has been designed to meet both FCC
Part 68 (USA) and DOT CSA CS-03 Part I (Canada)
isolation and surge requirements when circuited as
indicated in Figure 5. Among the more critical
parameters, the PSTN isolation barrier will provide at
least 1500VAC RMS isolation and 2122V peak surge
protection. The CH1837A/7F/8A is also UL1950
Edition 3 listed and meets CSA C222-950 test
conditions when connected as indicated in Figure 5.
2002 Cermetek Microelectronics, Inc.
Page 2
Document No. 607-0014 Revision C2 (12/02)