CH1840 Datasheet, PDF (2/7 Pages) List of Unclassifed Manufacturers – Low Profile Data Access Arrangement (DAA) Modules

Drawing current from the local loop signals the central office that
the telephone is Off Hook, that is, either answering a call or prepar-
ing to place a call. In the CH1840, this function is controlled by the
OFFHK signal. When this input is LOW, the CH1840 is On-Hook,
indicating to the central office it is not using the line. The CH1840
can pulse dial by toggling the OFFHK input.
For data calls, FCC Part 68 rules require silence on the phone line
for at least 2 seconds after the call has been completed to allow
central office to exchange billing information. The CH1840 as-
sures conformance to this requirement by starting a two-second
timer on the LOW to HIGH transition to OFFHK and squelching
transmitted signals until the time expires. The CH1840 defeats
this function when Voice Mode is selected on the Data/Voice pin.
Ring Detection
The central office signals an incoming call by placing a ring signal
on the local loop. The CH1840 monitors the loop for this signal.
The Ring Indication output, RI, is normally HIGH. When ringing
appears, RI is set LOW during the typically 2 second ring period
and is restored to HIGH for the typically 4 seconds between rings.
The ring detection circuit is designed to prevent false indications
due to noise or pulse dialing on the line.
The RI output of the CH1840 is diode protected so that an external
pull-up resistor (R> 100Kohm) to +5V may be utilized to activate
the ring detection circuit when the DAA is not connected to power.
This can be handy with 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. See Figure 3.
Transmit Signal
The outgoing analog signal to be transmitted through the phone
line should be applied to the XMIT pin (with respect to GND) and
must be AC coupled, as shown in Figure 2. For the CH1840D, a
XMIT(+) and XMIT(-) pin designation provides a differential input.
Figure 3. Low Power Ring Detection
The CH1840 attenuates the transmit signal by 9.5 dB, 0 dB for the
D option). Thus, a transmit signal of 0 dBm for the CH1840 ( -9.5
dBm for D) applied to XMIT will comply with the FCC Part 68 re-
quirement for data signals of -9dBm across Tip and Ring. In all
cases in the data mode, the CH1840 prevents the signal level
from being greater than -9 dBm at Tip and Ring.
For live voice or DTMF, when the CH1840 family products are in
voice mode, the output can be 0 dBm. In voice mode, the squelch
will not let the output signal exceed 0 dBm. Hence, by applying a
0 dBm signal to the CH1840D and a +9.5 dBm signal to the
CH1840, it will achieve the maximum signal output.
CH1840 Family
CH1840
CH1840D
Nominal Insertion Loss
-9.5 dB
0 dB
Figure 2. Typical Application
© 2000 Cermetek Microelectronics, Inc.
Page 2
Document No. 607-0011 Rev. A (5/00)