PWM-input to 12-bit-accurate voltage output DAC

August 13, 2014 // By Graham Prophet
No software, no ripple and no delay are the key claims for Linear Technology’s LTC2645, a quad-channel 12-bit/10-bit/8-bit PWM-to-voltage output digital-to-analogue converter (DAC) with 10ppm/°C reference.

These parts convert PWM input signals to 12-bit accurate, stable, buffered voltage outputs in less than 8 µsec, eliminating the ripple and delay typically associated with analogue filters. The LTC2645 provides a simple bolt-on solution to accurately set and adjust four voltages without a single line of code required. In a 16-lead MSOP package, the LTC2645 is suitable for biasing, calibration, level setting, power supply adjustment, and to enable opto-isolated communications in applications such as industrial automation, 4-20 mA process control, instrumentation, test equipment and optical modules. A dual version, the LTC2644, is available in a 12-lead MSOP package.

The LTC2645 measures the period and pulse width of the PWM input signals and updates the DACs after each PWM input rising edge, accepting PWM input frequencies from 30 Hz up to 100 kHz. An IDLSEL pin provides flexibility to set the outputs to idle at zero- or full-scale, power-down with high-impedance output, or hold the previous state indefinitely in response to an idle PWM input. This convenient mode has the advantage over analogue filter implementations, which require the PWM to run continuously. Each rail-to-rail DAC output is capable of sourcing or sinking 5 mA (3V) or 10 mA (5V), and offers a full-scale output of 2.5V using the internal 10 ppm/°C reference or a full-scale output equal to the external reference.

The LTC2645 operates from a single 2.7V to 5.5V supply, and supports PWM input voltages from 1.71V to 5.5V. The device consumes 4 mA from a 3V supply, and has a power-down mode with high impedance outputs in which it uses under 1 µA. It achieves ±2.5 LSB Max INL; ±1LSB Max DNL, and is guaranteed monotonic.

The LTC2645 is offered in 12-bit/10-bit/8-bit versions in commercial, industrial and automotive (-40°C to 125°C) temperature grades. Pricing begins at $3.95 for the LTC2645-12 (1000). The DC2197A evaluation board for the LTC2645 family is supported by the Linduino Firmware Development System, using