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DAC8802 Datasheet(PDF) 11 Page - Texas Instruments |
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DAC8802 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 18 page ![]() www.ti.com THEORY OF OPERATION CIRCUIT OPERATION Digital-to-Analog Converters R R R 2R 2R 2R R 5 kΩ S2 S1 DGND VREFX VDD RFBX IOUTX AGNDX Digital interface onnections are omitted for clarity. Switches S1 and S2 are closed; V must be powered. DD c V OUT + *VREF D 16384 (1) DAC8802 SBAS351A – AUGUST 2005 – REVISED DECEMBER 2005 The DAC8802 contains two 14-bit, current-output, digital-to-analog converters (DACs). Each DAC has its own independent multiplying reference input. The DAC8802 uses a 3-wire, SPI-compatible serial data interface, with a configurable asynchronous RS pin for half-scale (MSB = 1) or zero-scale (MSB = 0) preset. In addition, an LDAC strobe enables two-channel simultaneous updates for hardware-synchronized output voltage changes. The DAC8802 contains two current-steering R-2R ladder DACs. Figure 30 shows a typical equivalent DAC. Each DAC contains a matching feedback resistor for use with an external I-to-V converter amplifier. The RFBX pin is connected to the output of the external amplifier. The IOUTX terminal is connected to the inverting input of the external amplifier. The AGNDX pin should be Kelvin-connected to the load point in the circuit requiring the full 14-bit accuracy. Figure 30. Typical Equivalent DAC Channel The DAC is designed to operate with both negative or positive reference voltages. The VDD power pin is only used by the logic to drive the DAC switches on and off. Note that a matching switch is used in series with the internal 5 k Ω feedback resistor. If users are attempting to measure the value of R FB, power must be applied to VDD in order to achieve continuity. The DAC output voltage is determined by VREF and the digital data (D) according to Equation 1: Note that the output polarity is opposite of the VREF polarity for dc reference voltages. The DAC is also designed to accommodate ac reference input signals. The DAC8802 accommodates input reference voltages in the range of -15 V to +15 V. The reference voltage inputs exhibit a constant nominal input resistance of 5 k Ω, ±20%. On the other hand, DAC outputs I OUTA and B are code-dependent and produce various output resistances and capacitances. The choice of external amplifier should take into account the variation in impedance generated by the DAC8802 on the amplifiers' inverting input node. The feedback resistance, in parallel with the DAC ladder resistance, dominates output voltage noise. For multiplying mode applications, an external feedback compensation capacitor (CFB) may be needed to provide a critically damped output response for step changes in reference input voltages. 11 |
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