CH1837A Datasheet

Data Access Arrangements


Features, Applications

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 is a special order product only. Consult Cermetek for availability and delivery.


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.

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.


Low Profile Complete DAA function Compatible with most popular V.34bis modem chip sets Ring detection circuitry included Built-in 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.32, V.22bis, and V.22 FCC Part 68 (USA) and DOT CSA CS-03 Part I (Canada) Approvable

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 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.

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
Opt coupler, 30K pull-up, Active LOW Square wave 15-68HZ (TIP. 20Hz) 38Vrms across TIP and RING

Figure 4. Low Power Ring Detection Circuit. 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.

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.

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 0.1�F 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 0.1�F capacitor as shown in Figure 3. In single ended signal operation, the XMIT(+) pin must be connected to GND through a 0.1�F 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.

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

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.

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 -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 0.1�F 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:

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

The CH1837A/7F/8A DAA Module contains staticsensitive components and should only be handled by



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