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DAC729JH Datenblatt(PDF) 5 Page - Burr-Brown (TI) |
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DAC729JH Datenblatt(HTML) 5 Page - Burr-Brown (TI) |
5 / 12 page DAC729 5 THEORY OF OPERATION The DAC729 is an 18-bit digital-to-analog converter sys- tem, including a precision reference, low noise, fast settling operational amplifier, and an 18-bit current source/DAC chip contained in a hermetic 40-pin ceramic dual-in-line package. Refer to Figure 11 for a schematic diagram of the DAC729. THE INTERNAL REFERENCE The reference consists of a very low temperature coefficient closed-loop reference zener circuit that has been tempera- ture-drift-compensated by laser-trimming a zener current to achieve less than 1ppm/ °C temperature drift of V REF. By strapping pin 32 (Reference Out) to pin 31 (Reference In), the DAC will be properly biased from the internal reference. The internal reference may be fine adjusted using pin 35 as shown in Figure 7. The reference has an output buffer that will supply 4mA for use external to the DAC729. This load must remain constant because changing load on the reference may change the reference current to the DAC. In systems where several components need to track the same system reference, the DAC729 may be used with an external 10V reference, however, the internal reference has lower noise (6 µVp-p) and better stability than other references available. THE OPERATIONAL AMPLIFIER To support a DAC of this accuracy, the operational amplifier must have a maximum gain-induced error of less than 1/3LSB, independent of output swing (the op amp must be linear!) To support 15 bits (1/2-bit linearity), the op amp must have a gain of 130,000V/V. For 18 bits, the minimum DAC ANALOG OUTPUT DIGITAL INPUT COB 20V FSR CSB 10V FSR 00 0000 0000 0000 0000 + Full Scale 9.999924V + Full Scale 9.999962V 11 1111 1111 1111 1111 – Full Scale –10V – Full Scale 0V TABLE I. Digital Input Coding. BURN-IN SCREENING Burn-in screening is an option available for the DAC729 family of products. Burn-in duration is 160 hours at 100 °C (or equivalent combination of time and temperature). All units are tested after burn-in to ensure that grade speci- fications are met. gain is well over 500,000V/V. Since thermal feedback is the major limitation of gain for mono op amps, the amplifier was designed as a high gain, fast settling mono op amp, followed by a monolithic, unity-gain current buffer to isolate the thermal effects of external loads from the input stage gain transistors. The op amp and buffer are separated from the DAC chip, minimizing thermally-induced linearity er- rors in the DAC circuit. The op amp, like the reference, is not dedicated to the DAC729. The user may want to add a network, or select a different amplifier. The DAC729 inter- nal op amp is intended to be the best choice for accuracy, settling time, and noise. THE DAC CHIP The heart of the DAC729 is a monolithic current source and switch integrated circuit. The absolute linearity, differential linearity, and the temperature performance of the DAC729 are the result of the design, which utilizes the excellent element matching of the current sources and switch transis- tors to each other, and the tracking of the current setting resistors to the feed back resistors. Older discrete designs cannot achieve the performance of this monolithic DAC design. The two most significant bits are binarily weighted inter- digitated current sources. The currents for bits 3 through 18 are scaled with both current source weighting and an R-2R ladder. The circuit design is optimized for low noise and low superposition error, with the current sources arranged to minimize both code-dependent thermal errors and IR drop errors. As a result, the superposition errors are typically less than 20 µV. The DAC chip is biased from a servo amplifier feeding into the base line of the current sources. This servo amplifier sets the collector current to be mirrored and scaled in the DAC chip current sources, as shown in Figure 11. The reference current for the servo is established by the reference voltage applied to pin 31 feeding an internal resistor (20k Ω) to the virtual ground of the servo amplifier. DISCUSSION OF SPECIFICATIONS DIGITAL INPUT CODES The DAC729 accepts complementary digital input codes in either binary format (CSB for Unipolar or COB for Bipolar; see Table I). ELECTROSTATIC DISCHARGE SENSITIVITY Any integral circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degrada- tion to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet published specifications. |
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