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發表於 2014-1-17 13:20:38
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considerations re dac design
The AD5791 Is Only the Beginning:
1-ppm Circuit Complexities
Even though precision sub-1-ppm components such as the AD5791 are available on the market, building a 1-ppm system is not a task that should be taken lightly or rushed into. Error sources that show up at this level of precision must be carefully considered. The major contributors to errors in 1-ppm-accurate circuits are noise, temperature drift, thermoelectric voltages, and physical stress. Precision circuit construction techniques should be followed to minimize the coupling and propagation of these errors throughout the circuit and the introduction of external interference. These considerations will be summarized here briefly. Further information can be found in the References.
Noise
When operating at 1-ppm resolutions and accuracies, it is of utmost importance to keep noise levels to a minimum. The noise spectral density of the AD5791 is 9 nV/√Hz, mostly from the Johnson noise of the 3.4-kΩ DAC resistance. All peripheral components should have smaller noise contributions to minimize increases to the system noise level. Resistor values should be less than the DAC resistance to ensure that their Johnson noise contribution will not significantly add to the root-sum-square overall noise level. The AD8676 reference buffers and the AD8675 output buffer have a specified noise density of 2.8 nV/√Hz, well below the DAC's contribution.
High-frequency noise can be eliminated relatively easily with simple R-C filters, but low-frequency 1/f noise in the 0.1-Hz to 10-Hz range cannot be easily filtered without affecting dc accuracy. The most effective method of minimizing 1/f noise is to ensure that it is never introduced into the circuit. The AD5791 generates about 0.6 μV p-p of noise in the 0.1-Hz to 10-Hz bandwidth, well below the 1-LSB level (1 LSB = 19 μV for a ±10-V output span). The target for maximum 1/f noise in the entire circuit should be about 0.1 LSB, or 2 μV; this can be ensured through proper component choice. The amplifiers in the circuit generate 0.1-μV p-p 1/f noise; the three amplifiers in the signal chain generate a total of approximately 0.2-μV p-p noise at the circuit output. Add this to the 0.6-μV p-p from the AD5791, and the total expected 1/f noise is about 0.8 μV p-p, a figure that closely correlates with the measurement displayed in Figure 5. This offers adequate margin for other circuitry that may be added, such as amplifiers, resistors, and a voltage reference.
Besides random noise, it is important to avoid errors caused by radiated, conducted, and induced electrical interference. Such techniques as shielding, guarding, and scrupulous attention to grounding and proper printed-circuit-board wiring techniques are imperative.
Temperature Drift
As with all precision circuits, drift of all components with temperature is a major source of error. The key to minimizing the drift as much as possible is to choose critical components with sub-1-ppm temperature coefficients. The AD5791 exhibits a very low 0.05-ppm/°C temperature coefficient. The AD8676 reference buffers drift at 0.6 μV/°C, introducing an overall 0.03-ppm/°C gain drift into the circuit; the AD8675 output buffer contributes a further 0.03-ppm/°C output drift; this all adds up to a figure of 0.11 ppm/°C. Low drift, thermally matched resistor networks should be used for scaling and gain circuits. Vishay bulk metal-foil voltage-divider resistors, series 300144Z and 300145Z, with a temperature coefficient of resistance tracking to 0.1 ppm/°C, are recommended.
Thermoelectric Voltages
Thermoelectric voltages are the result of the Seebeck effect: temperature-dependent voltages are generated at dissimilar metal junctions. Depending on the metallic components of the junction, the generated voltage can be anywhere from 0.2 μV/°C to 1 mV/°C. The best case, a copper-to-copper junction, will generate less than 0.2 μV/°C of thermoelectric EMF. In the worst case, copper-to-copper-oxide can generate up to 1 mV/°C of thermoelectric voltage. This sensitivity to even small temperature fluctuations means that nearby dissipative elements or slow-moving air currents crossing over a printed circuit board (PCB) can create varying temperature gradients, which in turn generate varying thermoelectric voltages that are manifested as a low-frequency drift similar to low-frequency 1/f noise. Thermoelectric voltages can be avoided by ensuring that there are no dissimilar junctions in the system and/or eliminating thermal gradients. While it is virtually impossible to eliminate dissimilar metal junctions—many different metals exist in IC packaging, PCB circuits, wiring, and connectors—keeping all connections clean and oxide-free will go a long way to keeping thermoelectric voltages low. Enclosing the circuit to shield circuitry from air currents would be an effective thermoelectric voltage stabilizing method, and it could have the added value of providing electrical shielding. Figure 7 shows the difference in voltage drifts between a circuit that is open to air currents and one that is enclosed.
Figure 7. Voltage drift vs. time for open- and enclosed systems.
To cancel out the thermoelectric voltages, compensating junctions could be introduced into the circuit, a task that would involve considerable trial and error and iterative testing to ensure the correct pairing and positions of the inserted junctions. By far the most efficient method is to reduce the number of junctions in the circuit by minimizing component count in the signal path and stabilizing the local and ambient temperatures.
Physical Stress
High-precision analog semiconductor devices are sensitive to stress on their package. Stress relief compounds used within the packaging have a settling effect, but they cannot compensate for significant stress due to pressure exerted directly on the package by local sources, such as flexing of the PCB. The larger the printed circuit board, the more stress that a package could potentially suffer, so sensitive circuitry should be placed on as small a board as possible—with connection to the larger system through flexible or nonrigid connectors. If a large board cannot be avoided, stress relief cuts should be made around sensitive components, on two or (preferably) three sides of the component, greatly reducing the stress on the component due to board flexing.
Long-Term Stability
After noise and temperature drift, long-term stability deserves consideration. Precision analog ICs are very stable devices but do undergo long-term age-related changes. Long-term stability for the AD5791 is typically better than 0.1 ppm/1000 hours at 125°C. The aging is not cumulative but follows a square root rule (if a device ages at 1 ppm/1000 hours, it ages at √2 ppm/2000 hours, √3 ppm/3000 hours, ...), and the time is typically 10 times longer for each 25°C reduction in temperature; so, at 85°C operation, one can expect aging of 0.1 ppm over a 10,000 hour period, approximately 60 weeks. If this is extrapolated, 0.32 ppm aging can be expected over a 10-year period, so the data sheet dc specifications can be expected to drift by 0.32 ppm over a 10-year period when operating at 85°C.
Circuit Construction and Layout
In a circuit where such a high level of accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. Design the PCB such that the analog and digital sections are separated and confined to separate areas of the board. If the DAC is in a system where multiple devices require an analog-to-digital ground connection, establish the connection at one point only. Establish the star-point ground as close as possible to the device. There should be ample power supply bypassing of 10 μF in parallel with 0.1 μF on each supply terminal, as close to the package as possible, ideally right up against the device. The 10-μF capacitors should be of the tantalum bead type. The 0.1-μF capacitor should have low effective series resistance (ESR) and low effective series inductance (ESL), such as the common multilayer ceramic types—to provide a low-impedance path to ground at high frequencies to handle transient currents due to internal logic switching. A series ferrite bead on each power supply line will further help to block high-frequency noise from getting through to the device.
The power supply traces should be as large as possible to provide low-impedance paths and reduce the effects of glitches on the power-supply line. Shield fast-switching signals, such as clocks, with digital ground to avoid radiating noise to other parts of the board. They should never be run near the reference inputs or under the package. It is essential to minimize noise on the reference inputs because it couples right through to the DAC output. Avoid crossover of digital and analog signals, and run traces on opposite sides of the board at right angles to each other to reduce the effects of feedthrough on the board.
Voltage Reference
Holding the performance of the entire circuit firmly within its grasp is the external voltage reference; its noise and temperature coefficient directly impact the system's absolute accuracy. To capitalize on the challenge posed by the 1-ppm AD5791 digital-to-analog converter, the reference and associated components should have temperature drift and noise specifications comparable to those of the DAC. Although a reference with temperature drift of 0.05 ppm/°C is nothing short of fantasy, 1 ppm/°C and 2 ppm/°C voltage references with 0.1-Hz to 10-Hz noise of less than 1 μV p-p do exist.
Conclusion
As the accuracy requirements of precision instrumentation—and test and measurement applications—increase, more accurate components are being developed to meet these needs. They have guaranteed precision specifications at the 1-ppm level without further user calibration and are easy to use. However, when designing circuitry for this level of precision, one must bear in mind the many environmental and design-related challenges that exist. Successful precision-circuit performance will come as a result of considering and understanding these challenges and making correct component choices.
source:
http://www.analog.com/library/an ... s/44-04/ad5791.html |
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發表於 2014-1-15 15:19:24
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