選擇合適的系列電壓基準源的絕對精度電壓輸出
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(1%,100ppm/°C, 400μA)
MAX6102*
(0.4%, 75ppm/°C, 5mA)
MAX6125
(1%, 50ppm/°C, 1mA)
(0.04/0.12%, 2/5ppm/°C,5mA)
MAX6325*
(0.04%, 1ppm/°C, 15mA)
(0.1/0.24%, 5/10ppm/°C, 5mA)
MAX6191A/B/C
(0.1/0.24/0.5%, 5/10/25ppm/°C, 500μA)
(0.32/0.48%, 20/30ppm/°C, 500μA)
MAX6190A*/B/C
(0.16/0.32/0.48%, 5/10/25ppm/°C, 500μA)
*Initial reference selection for each design example
Design A: Low Cost, Loose Accuracy
At first glance, the MAX6002 ($0.39/2500pcs) appears to be an obvious choice for Design A, which requires low cost and fairly loose accuracy. But a further look reveals that the MAX6002 is not a good option. Its combined initial accuracy (1%, which is ~10LSB at 10 bits) and tempco error (70°C × 100ppm/°C = 7000ppm ~ 7LSB at 10 bits) already violate the overall accuracy requirement of Design A (17LSB exceeds the design requirement of 16LSB at 10 bits), even without including the other error terms such as load regulation, noise, and so forth. The MAX6125 also has 1% accuracy, and its 50ppm/°C tempco brings us within the Design-A error limit (~13.5LSB), but its cost ($0.95/1000pcs) is too high for this application.The MAX6002 could be used if the accuracy requirement is loosened further or if some type of calibration scheme is implemented, whereas the more expensive MAX6125 could probably meet the requirements without compromise. Assuming the accuracy requirement is rigid, this example illustrates a key trade-off in reference selection for DAC designs: initial component cost (the MAX6125) versus cost of calibration (the MAX6002).
Further study of the Maxim voltage-reference selection table reveals a better option for Design A. If we arbitrarily allocate half of the total error (8LSB at 10 bits ~ 0.8%) to the initial accuracy and half to the tempco (0.8% = 8000ppm/70°C = 114ppm/°C), the MAX6102 ($0.55/2500pcs, 0.4% initial accuracy, 75ppm/°C) surfaces as the best choice. The MAX6102 can source 5mA of current to the load, so it is able to drive the MAX5304 DAC's reference input (2.5V/18 kΩ ~ 140μA max). If the MAX6102 is ruled out when the other error terms are analyzed, the MAX6125 can be reconsidered as a backup alternative.
Design B: High Absolute Accuracy and Precision
Because Design B has such challenging accuracy requirements, the MAX6225 and MAX6325 buried-zener references are the initial candidates because they have such low tempcos, excellent long-term stability, and low noise. These devices also have very good initial accuracy, but this is a moot specification in the case of Design B, as gain errors caused by the DAC and the voltage reference are calibrated out. The MAX6225 and the MAX6335 source 15mA, so driving the MAX5170 DAC reference input (2.5V/18k ~ 140μA max) is not an issue. The MAX6325 is chosen because it has the only tempco (70°C × 1ppm/°C = 70ppm max) that puts us beneath the overall 122ppm accuracy requirement (2LSB @ 14 bits = 2/214 = 2/16384 = 1.22 × 10-4 = 122ppm) while leaving margin for the other error sources. If we relax the Design-B accuracy requirements slightly, the MAX6225 A-grade device (2ppm/°C max tempco) would allow us to cut the reference cost by more than half.A 12V supply is conveniently present in the Design-B example, allowing the use of the MAX6325, which needs an input voltage of at least 8V. If 8V (or higher) is not available in the system, the MAX6166 (A grade) or MAX6192 (A grade) bandgap-based references could be considered, but a slight relaxation of the system specifications would be required.
Design C: One-Time Calibrated, Low Drift
The MAX6162 and MAX6191 A-grade devices are considered because of their low tempcos (5ppm/°C max), which are necessary to meet the requirements of Design C:Total Error Budget is 4LSB at 12 bits = 4 / 4096 ×106 = 977ppmNote that the MAX6162 and the MAX6191 both have 2mV (977ppm) initial accuracy, but this is not a concern with the 2.048V reference because the output-voltage range is only 0-4.000V and a gain calibration is planned for this design. The MAX6162 (5mA of output current drive) and the MAX6191 (500μA of output current drive) are both capable of driving the 293μA reference input current that results when the two MAX5154 DAC reference pins are tied together (2.048V/[14kΩ||14kΩ]); however, the MAX6162 has more margin if additional loads are connected to the reference output. The MAX6162 does have higher quiescent current than the MAX6191 (120μA versus 35μA max), but this is not a deciding factor as Design C is not power-constrained.
Required Tempco = 977ppm / (85 - (-40)) °C = 7.8ppm / °C
Available Error Beyond Tempco = 977ppm - 5ppm / °C ×125°C = 352ppm
After reviewing the initial specifications, it's clear that either device is probably acceptable. However, the MAX6162 is the first choice due to its higher output current. If further analysis shows the MAX6162 to be marginally unacceptable, the MAX6191 could be considered because it has slightly better load-regulation, temperature-hysteresis, and long-term-stability specifications.
Design D: Low Voltage, Battery Powered, Moderate Accuracy
Following the approaches used in the other examples, the total error for Design D is found to be 3906ppm (106 × 16/4096). Over the narrow 15°C to 45°C temperature range, we can tolerate a tempco of at most 130.2ppm/°C (3906ppm/30°C). Using our rule of thumb from Design A to allocate roughly half of the error budget to the tempco (65ppm/°C), reasonable, conservative reference choices are the MAX6012 (A and B grades are 20ppm/°C and 30ppm/°C, respectively) and the MAX6190 (A, B, and C grades are 5ppm/°C, 10ppm/°C, and 25ppm/°C, respectively). These parts are considered, because they have a maximum quiescent current of 35μA, which is appropriate for the low-power needs of Design D.The MAX6190 price (C grade is $1.45/1000pcs) is in the same range as the MAX6012 price (B grade is $1.35/2500pcs). Either part will probably work in the application. However, the A-grade MAX6012 is especially attractive, because it is available in a SOT23-3 package, which is ideal for a small, battery-powered, portable instrument.
A quick check of the A-grade MAX6012 reveals the tempco-related error to be 600ppm (30°C × 20ppm/°C). The initial error of 3200ppm (0.32%) also needs to be considered, as there is no trimming planned for this design. The sum of these two errors is 3800ppm out of the possible 3906ppm design limit. With this marginal situation, it's likely that some of the other specifications considered in the next section (Step 3), such as load regulation, temperature hysteresis, and even line regulation (because of the varying battery voltage), will put us beyond 3906ppm. Because the MAX6012 is probably insufficient, we will forego the SOT23-3 package and choose the A-grade MAX6190 as a starting point, as its initial error of 1600ppm and 5ppm/°C leave enough room for the other error terms. Reference output current is not a concern for this design, because the MAX6190 can supply 500μA (>>69μA design requirement).
Step 3: Final Specification Review and Error-Budget Analysis
With the preliminary selection of references complete and backup ICs in place, it's now time to verify the remaining specifications, which include reference-load regulation, input-line regulation, output-voltage temperature hysteresis, output-voltage long-term stability, and output noise voltage. The key system-level and DAC specifications for each design are also needed for the analysis (Table 4).Table 4. Important Specifications for Final Analysis
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