14-bit/16-bit multiplying DAC
Output control on power-up and power-down internal or
Versatile serial interface
DAC clears to 0 V in both unipolar and bipolar output ranges
The AD7849 is a 14-bit/16-bit serial input multiplying digital- to-analog converter (DAC). The DAC architecture ensures excellent differential linearity performance, and monotonicity is guaranteed to 14 bits for the A grade and to 16 bits for all other grades over the specified temperature ranges. During power-up and power-down sequences (when the supply voltages are changing), the VOUT pin is clamped to 0 V via a low impedance path. To prevent the output of A3 from being shorted to 0 V during this time, Transmission Gate G1 is also opened. These conditions are maintained until the power supplies stabilize, and a valid word is written to the DAC register. At this time, G2 opens and G1 closes. Both transmission gates are also externally controllable via the reset in (RSTIN) control input. For instance, if the RSTIN input is driven from a battery supervisor chip, then at power-off or during a brown out, the RSTIN input is driven low to open G1 and close G2. The DAC must be reloaded, with RSTIN high, to reenable the output. Conversely, the on-chip voltage detector output (RSTOUT) is also available to the user to control other parts of the system. The AD7849 has a versatile serial interface structure and can be controlled over three lines to facilitate opto-isolator applications. SDOUT is the output of the on-chip shift register and can be used in a daisy-chain fashion to program devices in the multi- channel system. The daisy-chain enable (DCEN) input controls this function. The BIN/COMP pin sets the DAC coding; with BIN/COMP set to 0, the coding is straight binary; and with BIN/COMP set to 1, the coding is twos complement. This allows the user to reset the DAC to 0 V in both the unipolar and bipolar output ranges. The part is available in a 20-lead PDIP package and a 20-lead SOIC package.