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MAX512EPD Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX512EPD Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 16 page Low-Cost, Triple, 8-Bit Voltage-Output DACs with Serial Interface _______________________________________________________________________________________ 9 _______________Detailed Description Analog Section The MAX512/MAX513 contain three 8-bit, voltage-out- put, digital-to-analog converters (DACs). The DACs are “inverted” R-2R ladder networks using complementary switches that convert 8-bit digital inputs into equivalent analog output voltages in proportion to the applied ref- erence voltages. The MAX512/MAX513 have two reference inputs: one is shared by DAC A and DAC B and the other is used by DAC C. These inputs allow different full-scale output voltages and different output voltage polarities for the DAC pair A/B and DAC C. The MAX512/MAX513 include output buffer amplifiers for DACs A and B and input logic for simple micro- processor (µP) and CMOS interfaces. The MAX512/MAX513 operate in either single-supply or dual-supply mode, as determined by VSS. If VSS is with- in approximately -0.5V of GND, single-supply mode is assumed. If VSS is below -1.5V, the devices are in dual- supply mode. Reference Inputs and DAC Output Range The voltage at REF_ sets the full-scale output of the DACs. The input impedance of the REF_ inputs is code dependent. The lowest value, approximately 12k Ω for REFC (8k Ω for REFAB), occurs when the input code is 01010101 (55hex). The maximum value of infinity occurs when the input code is zero. In shutdown mode, the selected DAC output is set to zero while the value stored in the DAC register remains unchanged. This removes the load from the reference input to save power. Bringing the MAX512/MAX513 out of shutdown mode restores the DAC output voltage. Because the input resistance at REF_ is code depen- dent, the DAC’s reference sources should have an out- put impedance of no more than 5 Ω. The input capaci- tance at the REF_ pins is also code dependent and typically does not exceed 25pF. The reference voltage on REFAB can range anywhere between the supply rails. In dual-supply mode, a posi- tive reference input voltage on REFAB should be less than (VDD - 1.5V) to avoid saturating the buffer ampli- fiers. The reference voltage includes the negative sup- ply rail. See the Output Buffer Amplifier section for more information. The REFC input accepts positive voltages up to VDD and should not be forced below ground. The absolute difference between any reference voltage and GND should not exceed 6V. Output Buffer Amplifiers (DAC A / DAC B) DAC A and DAC B voltage outputs are internally buffered. The buffer amplifiers have a rail-to-rail (VSS to VDD) output voltage range. In single-supply mode, the DAC outputs A and B are internally divided by two and the buffer is set to a gain of two, eliminating the need for a buffer input voltage range to the positive supply rail. In dual-supply mode, the DAC outputs are not attenuat- ed and the buffer is set to unity gain. Although only necessary for negative output voltages, the dual-supply mode may be used even if the desired DAC output voltage is positive. Possible errors associ- ated with the divide-by-two attenuator and gain-of-two buffers in single-supply mode are eliminated in dual- supply mode. In this case, do not use reference volt- ages higher than (VDD - 1.5V). DAC A’s output amplifier can source and sink up to 5mA of current (0.5mA for DAC B buffer). See the Total Unadjusted Error vs. Digital Code graph in the Typical Operating Characteristics for dual and single supplies. The amplifier is unity-gain stable with a capacitive load of 0.05µF (0.01µF for DAC B buffer) or greater. The slew rate is limited by the load capacitor and is typically 0.1V/µs with a 0.1µF load (0.01µF for DAC B buffer). Unbuffered Output (DAC C) The output of DAC C is unbuffered and has a typical out- put impedance of 24k Ω. It can be used to drive a high- impedance load, such as an op amp or comparator, and has 35µs typical settling time to 1/2LSB with a single 3V supply. Use DAC C if a quick dynamic response is required. Figure 1. DAC Simplified Circuit Diagram 2R 2R 2R 2R 2R RR R REF GND OUT SHOWN FOR ALL 1s ON DAC; DAC C IS NOT BUFFERED |
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