选择合适的系列电压基准源的绝对精度电压输出

Parameter	Design A	Design B	Design C	Design D
Main Design Objectives	Low cost, loose accuracy	High absolute accuracy and precision	One-time calibrated, low drift	Low voltage, battery powered, moderate accuracy
Example Application	Consumer audio device	Lab instrument	Digital offset and gain adjustment	Portable instrument
DAC	MAX5304, 10-bit single	MAX5170, 14-bit single	MAX5154, 12-bit dual	MAX5176, 12-bit single
DAC Output	Force/sense gain set to 1	Fixed gain = 1.638	Fixed gain = 2	Fixed gain = 1.638
Voltage Reference	MAX6102	MAX6325	MAX6162 A grade	MAX6190 A grade
Reference Voltage	2.5V	2.5V	2.048V	1.25V
Reference Initial Accuracy	0.4% or 4000ppm	Not critical due to gain calibration	Not critical due to gain calibration	0.16% or 1600ppm
Selected Reference Tempco (maximum)	75ppm°C	1ppm/°C	5ppm/°C	5ppm/°C
Reference-Load Regulation	0.9mV/mA	6ppm/mA	0.9mV/mA	0.5μV/μA
Temperature Range	0°C to 70°C (commercial)	0°C to 70°C (commercial)	-40°C to 85°C (extended)	15°C to 45°C ( commercial)
Signal BW	10Hz to 10kHz	DC to 1kHz	DC to 10Hz	DC to 10Hz
DAC Calibration	None	Annual (gain and offset)	One-time factory (gain and offset)	None
Max Error Target	15625ppm (16LSB @ 10 bits)	122ppm (2LSB @ 14 bits)	977ppm (4LSB @ 12 bits)	3906ppm (16LSB @ 12 bits)

Each example is analyzed, focusing on the specifications that apply to that particular design. The results of this analysis, along with the results of the previous section, are summarized in an error budget in Table 5.

It is most convenient to do the error-budget accounting in ppm, although this could be done equivalently in other units such as %, mV, or LSBs. It's important to apply the proper scaling and to use the proper normalization factor to get the correct error values. Reference error terms can be equivalently calculated relative to the reference voltage or the DAC output voltage. For example, if we assume a reference error of 2.5mV (noise, drift, etc.) and a reference voltage of 2.5V, we get the following:

Reference Output Error = 106 × 2.5mV / 2.5V = 1000ppm

If we assume that the DAC output amplifier has a gain of 2.0, both the error and the reference voltage are scaled, so we get the same result at the DAC output (5V full-scale range):

DAC Output Error = 106 × (2.5mV × 2) / (2.5V × 2) = 1000ppm

Table 5. Error-Budget Analysis

Parameter	Design A	Design B	Design C	Design D
Main Design Objectives	Low cost, loose accuracy	High absolute accuracy and precision	One-time calibrated, low drift	Low voltage, battery powered, moderate accuracy
Example Application	Consumer audio device	Lab instrument	Digital offset and gain adjustment	Portable instrument
Reference Initial Error	4000ppm	-	-	1600ppm
Reference/DAC Post- Calibration Error	-	0ppm	244ppm	-
Reference Tempco Error	5250ppm	70ppm	625ppm	150ppm
Reference Temperature Hysteresis	130ppm	20ppm	80ppm	75ppm
Reference Long-Term Stability	100ppm	30ppm	160ppm	100ppm
Reference Load-Regulation Error	50ppm	1ppm	129ppm	28ppm
Reference Line-Regulation Error	120ppm	0ppm	0ppm	58ppm
Reference Output Noise	17ppm	2ppm	5ppm	10ppm
DAC INL	3906ppm	61ppm	122ppm	488ppm
DAC Gain Error	1953ppm	0ppm	-	1953ppm
DAC Gain TC	70ppm	-	500	-
DAC Noise	-	1ppm	-	0ppm
Worst-Case Error	15596ppm	184ppm	1865ppm	4462ppm
Root Sum Square Error	7917ppm	100ppm	874ppm	2580ppm< /FONT>
Target Error	15625ppm	122ppm	977ppm	3906 ppm
Worst-Case Margin	29ppm	-62ppm	-888ppm	-556ppm
Root Sum Square Margin	7708ppm	22ppm	103ppm	1326ppm

Design A: Low Cost, Loose Accuracy

No calibration or trimming is planned for Design A, so the MAX6102 initial error of 4000ppm (or 0.4%) directly becomes part of the budget, as does the 5250ppm due to the voltage-reference tempco (70°C × 75ppm/°C). The typical MAX6102 output-voltage temperature-hysteresis specification is also used directly in the error budget (keeping in mind that this is a typical value if we find ourselves with a design having marginal accuracy). For output-voltage long-term stability, we assume twice the MAX6102 1000-hour specification (2 × 50ppm = 100ppm), which is fairly conservative, as it's usually much better after the first 1000 hours. A conservative estimate here at least partially offsets the typical specification used for temperature hysteresis.

To calculate the variation in reference voltage caused by load regulation, we need to know the worst-case range of currents that the voltage reference supplies to the DAC's reference input. In Step 2, we determined the maximum DAC reference current that the MAX6102 would have to drive: 140μA. The minimum current is close to 0, as the MAX5304 reference input is effectively an open circuit (several GΩ input impedance) when the DAC code value is 0. This means the total output-current variation that the MAX6102 sees is 140μA, and this value should be used for the load-regulation calculation: