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
Minimum Reference Input Resistance	18kΩ	18kΩ	7kΩ (14kΩ||14kΩ shared reference inputs)	18kΩ
Output Voltage	Range = 0 -2.5V	Range = 0 - 4.096V	Range = 0 -4.000V	Range = 0 -2.048V
DAC Output	Force/sense	Fixed gain = 1.638	Fixed gain = 2	Fixed gain = 1.638
Power Supply	5V(varying) 4.5V min 5.5V max	5V(constant) 4.95V min +12V available	5V(constant) 4.75V min 5.25V max	3V(varying VBATT) 2.7V min 3.6V max
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	Burn-in + annual (gain and offset)	One-time factory (gain and offset)	None
Maximum Error Target	16LSB @ 10 bits (6-bit accuracy)	2LSB @ 14 bits (13-bit accuracy)	4LSB @ 12 bits (10-bit accuracy)	8LSB @ 12 bits (9-bit accuracy)

Step 1: Voltage Ranges and Reference-Voltage Determination

The first consideration when selecting a voltage reference for a DAC application is to evaluate the supply-voltage and the DAC output-voltage ranges (Table 2). To simplify the design examples described above, DACs have already been chosen, so their output gain is not a variable we will trade off as in a real design.

Table 2. Voltage-Related Parameters for DAC Design Examples

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
Output Voltage	Range = 0 - 2.5V	Range = 0 - 4.096V	Range = 0 - 4.096V	Range = 0 -2.048V
Power Supply	5V(varying) 4.5V min 5.5V max	5V(constant) 4.95V min +12V available	5V(constant) 4.75V min 5.25V max	3V(varying VBATT) 2.7V min 3.6V max
Reference Voltage and DAC Gain Options for Desired Output Voltage	2.5V (gain = 1)* 2.048V (gain = 1.221) 1.25V (gain = 2)	2.5V (gain = 1.638)* 2.048V (gain = 2) 1.25V (gain = 3.277) 3.0V (gain = 1.365)	2.048V (gain = 2)* 1.25V (gain = 3.277) 3.0V (gain = 1.365)	1.25V (gain = 1.638)*
Dropout Voltage	2.00V	9.5V	2.70V	1.45V

*Reference voltage and DAC gain chosen for each design example

Design A: Low Cost, Loose Accuracy

For the Design-A example, VDD is 5V and the output range is 0-2.5V, so a 2.5V reference is used and the MAX5304 force/sense output is set to unity gain (OUT and FB pins shorted). A lower voltage reference could be used with a higher, externally set gain, but we have opted to save the two resistors for this low-cost design.

Design B: High Absolute Accuracy and Precision

A 2.5V reference is chosen for the Design-B example, as the MAX5170 gain is fixed at 1.638 and a final output-voltage range of 0-4.096V is required. If a lower reference voltage is desired for Design B, a MAX5171 DAC could be used and its output force/sense gain could be set higher than 1.638 with external resistors, as shown in Figure 4. Note that the minimum VDD level is 4.95V, so the highest reference voltage we could use is 4.95V - 1.4V = 3.55V, as the DAC reference input is limited to (VDD - 1.4V)*.

*This limit applies to all DACs mentioned in this article.
Figure 4. Design-B reference options: (a) 2.5V (chosen), (b) 2.048V, (c) 1.25V.

Design C: One-Time Calibrated, Low Drift

In the Design-C example, the MAX5154 has a fixed gain of 2, so a 2.048V reference provides a nominal 4.096V output at full-scale. This voltage must exceed our 4.000V design requirement, so that we can use a gain calibration to scale the voltage down to the 0V to 4V range. This design also has other reference-voltage options if the MAX5156 force/sense DAC is used. Note that the reference-input upper-limit voltage is 4.75V - 1.4V = 3.35V.

Design D: Low Voltage, Battery Powered, Moderate Accuracy

The minimum VDD is 2.7V in the Design-D example, so the largest reference voltage that could be used is 2.7V - 1.4V = 1.3V. For this example, a 1.25V reference satisfies the 0V to 2.048V output range, as the MAX5176 gain is 1.638. It's important that the worst-case reference voltage, including all error terms, remains below 1.3V, or the specification for the DAC reference input voltage will be exceeded.

Approximate dropout voltages were calculated for each of the design examples (Table 2). All of these voltages are well above the 200mV (or lower) dropout voltages typical of Maxim's voltage references. Because the upper reference input voltage of most Maxim DACs is limited to VDD - 1.4V, dropout voltages can normally be ignored with these designs if the DAC and the voltage reference use the same positive supply rail. The dropout voltages are approximate, because they were calculated without any error terms such as initial accuracy, but these errors are small compared to typical dropout voltages and can be ignored.

Step 2: Initial Voltage-Reference Device-Selection Criteria

There are many factors to consider when selecting the optimum reference for each design. To make the procedure manageable, candidate devices will be identified based on the reference voltage determined above, an estimate of required initial accuracy, an approximated temperature coefficient, and the reference output current needed for the chosen DAC (Table 3). Other factors such as cost, quiescent current, packaging, and a quick glimpse at the remaining specifications will be used to select a specific initial device for each design. The remaining specifications will be analyzed in the following section (Step 3) to determine if the devices satisfy the overall accuracy requirements.

Table 3. Initial Device-Selection Considerations