DC261A-A Analog Devices

A High Performance Differential Line Driver/Receiver for DSL Applications Demo Board DC261A-A Revision 1.1 July 12, 1999 By Tim Regan LTC Amplifier Applications This Demo Manual describes the versatile features and measured performance of the DC261A demo board. Key features of this demo board include: High Speed, Low Distortion Differential Line Driver using the LT1795 Low Noise Differential 4 wire to 2 wire Hybrid Receiver using the LT1358 Single or Dual Supply Operation Adjustable Line Driver Operating Current for Optimum Power Dissipation External Driver ON/OFF Control Direct Phone Line Interface through 2:1 Transformer Low Thermal Resistance (22C/W junction to ambient) PCB Layout Introduction The schematic for the system is shown in Figure 1. The Line Driver is amplifier U1 A and B, an LT1795. The LT1795 is a Dual, 50MHz, 900V/mS current feedback amplifier with 500mA output current capability. The high output current drive allows for the use of higher turns ratio line coupling transformers with lower total supply voltage for the driver. The LT1795 has a wide 20-pin surface mount package with an enhanced power dissipating lead frame. These amplifiers are sufficient for the most demanding ADSL applications. Jumpers configure the line driver for operation as either a differential amplifier with a gain of 2, or as two separate non-inverting amplifiers, each with gain of 2. Provision has been made to allow adding an external resistor, at points TP3 and TP4 to modify the gain as required. The Line Receiver function is implemented by U4 A and B, an LT1358. These amplifiers provide 25MHz Gain- Bandwidth, 600V/mS slew rate, low power consumption (2.5mA supply current per amplifier) and low noise (8nV/Hz). The differential receiver senses and amplifies the voltage across the line transformer back-termination resistors. The gain setting resistors are scaled in such a manner as to provide first order cancellation of the transmitted signals from the line drivers so that only the received signal from the line is amplified. Shutdown and Adjustable Operating Current The LT1795 has a unique shutdown feature that can also be used to control the operating current of the amplifiers. This provides total control over the quiescent current consumption and power dissipation of the line drivers. When the line is inactive an external CMOS or TTL compatible logic signal can shut down the drivers to only 200mA of quiescent current. This ON/OFF control is provided by U4, which is an LTC1440 CMOS voltage comparator with reference. The 1.2V reference is the ON/OFF threshold for the control signal applied at TP2. When commanded ON by a logic 1 input, the LT1795 drivers bias up very quickly and process signals within about 1mS. Figure 1. DC261A Schematic (on next Page) Linear Technology Confidential For Customer Use Only Page 1 of 19 Linear Technology Confidential For Customer Use Only Page 2 of 19