Resistors get temperature compensation

Dallas Semiconductor has announced what it claims are the first digital resistors that can automatically measure and compensate for thermal variations affecting laser behaviour in optical transceivers.

As signal speed continues to increase in next-generation optical communications equipment, the DS1847 and DS1848 Dual Temperature-Controlled NV Variable Resistors can help designer engineers in achieving precision and safety of laser activity.

Resistors are used in optical transceivers to control current to VCSELs (vertical cavity surface emitting lasers) or laser diodes. Thermal changes affect current, which affects laser operation. By compensating for the effects of temperature on current, the designer can guarantee that the laser always operates within specifications.

Product manager Patrick Fedele said, ‘The DS1847 and DS1848 ICs integrate dual, 256-position, linear taper resistors with a digital thermometer and EEPROM. The DS1848 has an extra 128 bytes of general-purpose EEPROM; otherwise the two chips are alike. Both devices eliminate the need for external temperature checks or manual compensation in high-speed transceivers. Once installed, they operate very simply, automatically adjusting current with temperature change. If wanted, the temperature readings can be made directly available to the system.’

The designer determines device-specific resistance characteristics at temperature intervals of 2°C and stores the data in on-chip EEPROM. In operation, the DS1847 and DS1848 measure temperature, check readings against an EEPROM look-up table and adjust resistance accordingly. All thermal data and control instructions are communicated over an industry-standard, 2-wire interface.

The DS1847 and DS1848 provide precision temperature-compensated resistance control that is ideally suited for optical transceivers in gigabit Ethernet, fiber channel, SDH and SONET applications. Both chips available in 14-pin TSSOP or die-sized, flip-chip packages. Both operate with either 3V or 5V supplies and within a temperature range of -40°C to +95°C.

The DS1847 is priced at $2.29 and the DS1848 at $2.57, both in the TSSOP package in quantities of 1,000.