[Design Application]
Carefully Compare Pros And Cons To Correctly Choose An IC Voltage Reference
A designer must balance such factors as cost, size, precision, and power consumption when selecting a voltage reference.
A digital system with 16-bit resolution, for example, has an LSB size of one in 65,536 (15.26 × 10−6 or 15.26 ppm). If the ADC is a 16-bit device with a full-scale input of 0 to 5 V, it can resolve inputs down to 1 LSB, approximately 76.3 µV.
To cope with noncorrectable errors like noise, the reference should contribute very low noise so that every bit of ADC resolution counts. Good choices for this purpose are the MAX6150 (35 µV p-p), MAX6250 (3 µV p-p), and MAX6350 (also 3 µV p-p). Each contributes less than 1 LSB of noise in a 16-bit measurement. One alternative is to oversample and average the measurements, but that approach has its own limitations. Plus, it consumes processor power and increases system cost.
Output-voltage temperature hysteresis (THYS) is another noncorrectable error. THYS is the change in output voltage at the 25°C reference temperature due to sequential but opposite temperature excursions (from hot to cold and then cold to hot).
With its amplitude directly proportional to the temperature excursion, THYS can be very troublesome. In many situations, circuit design and packaging of the voltage-reference IC make this error nonrepeatable. One such situation appears in a MAX6001 reference where the three-pin SOT23 package has a typical THYS of 130 ppm. But a similar IC reference (MAX6190) in the larger, more stable SO-8 package exhibits only 75 ppm of THYS.
Temperature drift can usually be accommodated because it's generally a very repeatable error. High-resolution systems typically require compensation anyway. In order to keep a 5-V, 16-bit system within ±1 LSB over the commercial temperature range (0° to 70°C, with a 25°C reference point), the reference drift must be better than 1 ppm/°C. For instance:
ΔV = 1 ppm/°C × 5 V × 45°C = 255 µV
This performance is acceptable for a 14-bit system operating over the commercial temperature range. But it wouldn't satisfy the 1-LSB requirements of a 16-bit system (Table 2).
Long-term stability (LTS) provides an indication of the extent to which latent die stress or ion migration exists in a package or family of devices. Furthermore, circuit-board cleanliness over the extremes of temperature and humidity can strongly impact this parameter. LTS is valid only at the reference temperature of 25°C.
Another troublesome parameter is one that specifies the ability of a voltage reference to source and sink current. Most applications require the reference to source current to the load. Yet, many references can't sink current. Consequently, the output voltage can drift due to IBIAS and leakage currents if they exceed the current-sink capability of the reference.
An even worse situation is choosing a voltage reference that can't source the necessary load current. Typical reference currents required by ADCs and DACs range from tens of microamps (MAX1110) to 10 mA maximum (AD7886). The MAX6101-MAX6105 references source 5 mA and sink 2 mA. For really heavy loads, the MAX-6225/MAX6241/MAX6250 references can source and sink 15 mA.
The challenge of system design lies in balancing the tradeoffs of cost, size, precision, power consumption, and the like. Although they entail a larger bill of materials, the more expensive component-based systems, when implemented properly, require less compensation and calibration after the design is in production.
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