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AD5626BRMZ Datasheet(PDF) 10 Page - Analog Devices |
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AD5626BRMZ Datasheet(HTML) 10 Page - Analog Devices |
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10 / 20 page  AD5626 Rev. 0 | Page 10 of 20 THEORY OF OPERATION The AD5626 is a complete, ready-to-use, 12-bit digital-to-analog converter (DAC). It contains a voltage-switched, 12-bit, laser- trimmed DAC, a curvature-corrected band gap reference, a rail-to-rail output op amp, a DAC register, and a serial data input register. The serial data interface consists of an SCLK, serial data in (SDIN), and a load strobe (LDAC). This basic 3-wire interface offers maximum flexibility for interface to the widest variety of serial data input loading requirements. In addition, a CS select is provided for multiple packaging loading and a power-on-reset CLR pin to simplify start or periodic resets. DAC SECTION The DAC is a 12-bit voltage mode device with an output that swings from the GND potential to the 2.5 V internal band gap voltage. It uses a laser trimmed, rail-to-rail ladder which is switched by N-channel MOSFETs. The output voltage of the DAC has a constant resistance independent of digital input code. The DAC output internally connects to the rail-to-rail output op amp. AMPLIFIER SECTION A low power consumption, precision amplifier buffers the DAC output. This amplifier contains a differential PNP pair input stage that provides low offset voltage and low noise, as well as the ability to amplify the zero-scale DAC output voltages. The rail-to-rail amplifier is configured with a gain of 1.6384 (= 4.095 V/2.5 V) to set the 4.095 V full-scale output (1 mV/LSB). See Figure 23 for an equivalent circuit schematic of the analog section. 2R 2R 2R R2 R 2R 2R R R1 VOUT BUFFER BAND GAP REFERENCE 2.5V SPDT N-CHANNEL FET SWITCHES RAIL-TO-RAIL OUTPUT AMPLIFIER AV = 1.638 (= 4.095V/2.5V) VOLTAGE SWITCHED 12-BIT RAIL-TO-RAIL CONVERTER Figure 23. Equivalent AD5626 Schematic of Analog Section The op amp has a 16 μs typical settling time to 0.01%. There are slight differences in settling time for negative slewing signals vs. positive slewing signals. See the oscilloscope photos in the Typical Performance Characteristics section of this data sheet. OUTPUT SECTION The rail-to-rail output stage of this amplifier is designed to provide precision performance when operating near either power supply. P-CH VDD VOUT AGND N-CH Figure 24. Equivalent Analog Output Circuit Figure 24 shows an equivalent output schematic of the rail-to- rail amplifier with its N-channel pull-down FETs that pull an output load directly to GND. The output sourcing current is provided by a P-channel pull-up device that can supply GND terminated loads, especially at the low supply tolerance values of 4.75 V. Figure 5 and Figure 6 provide information on output swing performance near ground and full-scale as a function of load. In addition to resistive load driving capability, the amplifier has also been carefully designed and characterized for up to 500 pF capacitive load driving capability. POWER SUPPLY The very low power consumption of the AD5626 is a direct result of a circuit design optimizing use of the CBCMOS process. By using the low power characteristics of the CMOS for the logic, and the low noise, tight matching of the complementary bipolar transistors, good analog accuracy is achieved. For power consumption sensitive applications, it is important to note that the internal power consumption of the AD5626 is strongly dependent on the actual logic input voltage levels present on the SDIN, CS, LDAC, and CLR pins. Because these inputs are standard CMOS logic structures, they contribute static power dissipation dependent on the actual driving Logic VOH and Logic VOL voltage levels. The graph in shows the effect on total AD5626 supply current as a function of the actual value of input logic voltage. Consequently, use of CMOS logic vs. TTL minimizes power dissipation in the static state. A VIL = 0 V on the SDIN, Figure 9 CS, and CLR pins provides the lowest standby power dissipation of 2.5 mW (500 μA × 5 V). As with any analog system, it is recommended that the AD5626 power supply be bypassed on the same PC card that contains the chip. Figure 10 shows the power supply rejection vs. frequency performance. This should be taken into account when using higher frequency, switched mode power supplies with ripple frequencies of 100 kHz and higher. One advantage of the rail-to-rail output amplifier used in the AD5626 is the wide range of usable supply voltage. The part is fully specified and tested over temperature for operation from 4.75 V to 5.25 V. If reduced linearity and source current capa- bility near full scale can be tolerated, operation of the AD5626 is possible down to 4.3 V. The minimum operating supply |
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