CRMET CH1837A, CH1838A, CH1837F Datasheet

CH1837A/7F/8A Data Access Arrangement Module

V.34bis High Speed DAA Module

INTRODUCTION

The Cermetek CH1837A/F Data Access Arrangement
(DAA) is designed to meet the performance
requirements of 33.6Kbps modems, such as V.34bis,
for embedded applications. The CH1837A/F meets or
exceeds the requirements for V.34 and can be used in
lower speed applications as well. The isolation voltage
and surge protection meets, as a minimum, North
American UL1950 Edition 3 (USA) and CSA C22.2
950 (Canada) requirements. Further, the CH1837A/F
has been tested to meet FCC Part 68 and is Canadian
DOT CSA CS-03 Part I approvable.

The CH1837A/F is compatible with all popular V.34bis
modem/fax chip sets. Application examples are
included in this datasheet. The CH1837A/F offers a
Quick-to-Market DAA solution for high performance
modem designs.

The CH1837A/F is exceptionally well suited for voice­over-data (VoIP) and other internet-based telephony
and data communication applications.

For those applications not requiring a two to four wire
hybrid converter circuit, the CH1838A provides all the
features of the CH1837A/F (except the hybrid
converter) while offering some cost savings. Note that
the CH1838A is a special order product only. Consult
Cermetek for availability and delivery.

FEATURES

• Low Profile

• Complete DAA function

• Compatible with most popular V.34bis modem chip
sets

• Ring detection circuitry included

• Built-in 2-wire to 4-wire conversion circuitry
(CH1837A/F only)

• UL1950 Edition 3 Listed and CSA C22.2 950
Compliant

• 1500 VAC RMS Isolation

• 2122V peak surge protection

• +5V low power operation

• Differential transmission, VoIP applications

• Compatible with V.34bis, V.32bis, V.32, V.22bis,
and V.22

• FCC Part 68 (USA) and DOT CSA CS-03 Part I
(Canada) Approvable

DESCRIPTION OF FUNCTIONAL BLOCKS
AND DISCUSSION OF BASIC OPERATIONS

Figure 1 contains a functional block drawing of the
CH1837A/F. Each CH1837A/F product consists of:

1) Isolation barrier.

2) Off-Hook Switch/Ring Detect circuit.

3) PSTN Line Surge and High Voltage Protection
circuitry.

4) 2-to-4 Wire Conversion circuit.

Vcc VND

ISOLATION

XMIT+

XMIT-

RCV

RI

OFFHK

T1

HYBRID

2W/4W

CONVERTER

T2

Figure 1. Function Block Diagram of CH1837A/F.

Figure 2 contains a functional block drawing of the
CH1838A. Each CH1838A product consists of:

1) Isolation Barrier.

2) Off-hook Switch/Ring Detect circuit.

3) PSTN Line surge and High Voltage Protection

Circuitry.

Vcc VND

ISOLATION

T2

N/C

T1

RI

OFFHK

N/C

Figure 2. Function Block Diagram of CH1838A.

SURGE AND

PROTECTION

CIRCUITRY

OFF HOOK

RING DET.

SURGE AND

PROTECTION

CIRCUITRY

OFF HOOK

RING DET.

TIP

RING

TIP

RING

2002 Cermetek Microelectronics, Inc. Page 1 Document No. 607-0014 Revision C2 (12/02)

Cermetek Microelectronics, Inc. CH1837A/7F/8A Data Access Arrangement Module

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

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

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.

RI Output: Opt coupler, 30KΩ pull-up, Active

LOW
Square wave 15-68HZ (TIP. 20Hz)
Sensitivity: 38Vrms across TIP and RING

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

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.

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)

Cermetek Microelectronics, Inc. CH1837A/7F/8A Data Access Arrangement Module

TRANSMIT SIGNALS

The CH1837A/7F/8A does not attenuate the transmit
signal before it applies the signal across tip and ring.
Therefore, a transmit signal of –9dBm applied to XMIT
will comply with the FCC Part 68 requirement for data
signals across Tip and Ring.

CH1837A/F. The user presents all outgoing transmit
signals to the XMIT(-) pin with respect to GND for
single ended operation or between XMIT(-) and
XMIT (+) for differential operation. This signal must be
AC coupled through a 0.1µF capacitor present on both
the XMIT(-) and XMIT(+) pins as shown in Figure 3.

For single-ended operation, the outgoing analog signal
is applied to the XMIT(-) pin with respect to GND. This
signal must be AC coupled using a 0.1µF capacitor as
shown in Figure 3. In single ended signal operation,
the XMIT(+) pin must be connected to GND through a

0.1µF capacitor.

CH1838A. All incoming and outgoing signals are
presented between T1 and T2. The user must provide
an external hybrid conversion circuit and/or control the
flow of the transmit and receive signals into/from the
CH1838A.

RECEIVE SIGNALS

The incoming analog signal appears across TIP and
RING. The CH1837A/7F/8A does not amplify the
receive signal. Receive signals can vary from a
maximum strength of -9dBm to below -50dBm.

CH1837A/F. The CH1837A/F derives the incoming
signal and presents it at RCV with respect to GND.
The user must AC couple RCV through a 0.1µF
capacitor as indicated in Figure 3.

CH1838A. The CH1838A presents the incoming
signal between T1 and T2. The user must provide an
external hybrid conversion circuit and/or control the
flow of the transmit and receive signals into/from the
CH1838A.

Attenuation: 0.0dN
Input Impedance: 150KΩ
Typical Input Signal: 0dBm or 0.775Vrms with the

Signal referenced to GND and
AC coupled

UL RECOGNITION

To further protect the end product containing the
CH1837A/7F/8A from field failures caused by large
transients present on poor quality PSTN lines and to
maintain UL recognition, an additional level of external

transient protection is required. Refer to Figure 5 for
mandatory additional protection circuitry.

DESIGN CONSIDERATIONS

The CH1837A/7F/8A DAA includes circuitry that
couples the modem signals to the PSTN line and
provides the FCC required voltage isolation and surge
protection when circuited per Figure 5 herein.
Cermetek offers the following end product design
guidelines as suggestions only. These guidelines are
intended to facilitate the usage of Cermetek DAA
modules and to assist the user with FCC certification
of their end product.

1. Mount the DAA in the final assembly so that it is
isolated from exposure to any hazardous voltages
within the assembly. Adequate separation and
restraint of cables and cords must be provided.

2. Connect to the PSTN line through a standard FCC
approved RJ-11C jack or equivalent. For RJ-11,
use the two center pins.

3. All PCB traces from the CH1837A/7F/8A TIP and
RING pins must be at least 0.1 inch spacing from
all other traces or other conducting material. The
purpose for this spacing is to maintain 1500 VC
isolation between the incoming PSTN line and the
other traces. PSTN related PCB Traces should
have a width of 0.020 inches or greater.

4. TIP and RING PCB traces should be as short as
possible and should be oriented to prevent direct
or induced coupling with other signals present on
the PCB.

5. The CH1837A/7F/8A processes signals in the
millivolt range. Even though the CH1837A/7F/8A
is designed to handle noise in the power supply,
steps should be taken to assure that the noise
level does not exceed 50 mV peak-to-peak.
Particular attention should be paid to both the
power supply and the ground line PCB traces.

6. For data calls, FCC Part 68 rules require silence
on the PSTN line for at least 2 seconds after a
data call has been completed to allow central
offices to exchange billing information. Further,
Part 68 specifies that the transmit level must not
exceed -9dBm.

7. For voice calls, the FCC rules require that the end
product must meet the requirements of Part 68 for
Out-of-Band Energy, and DTMF Transmit Levels.

2002 Cermetek Microelectronics, Inc. Page 3 Document No. 607-0014 Revision C2 (12/02)