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CH1837A

Data Access Arrangement Module V.34bis High Speed DAA Module

Cermetek Microelectronics 

Cermetek Microelectronics, Inc.

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:

Input Impedance:

Typical Input Signal:

0.0dN

150KΩ

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

CH1837A/7F/8A Data Access Arrangement Module

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)

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