Because most of today's digital logic, mixed-signal, and analog circuits include a voltage reference, designers must understand how references operate and how to choose from among the available types. From the many specifications that characterize a voltage reference, it's important to know which are necessary for a given application, and why they fill the need.
The 1s and 0s of digital circuits, for example, are distinguished by thresholds that correspond to a logic high or logic low. Such applications don't require discrete-component zener or band-gap references. Instead, the 1s and 0s are determined by a "reference" consisting of the internal base-emitter voltage drop (VBE) of a bipolar transistor in TTL circuits, or by the gate-source voltage drop (VGS) of a MOSFET in CMOS circuits.
In a mixed-signal device, like the ubiquitous analog-to-digital converter (ADC) or digital-to-analog converter (DAC), the voltage reference can be included on the chip or provided externally. For purely analog circuits, such as a linear regulator, the output voltage is maintained at the desired level by comparing the known value of a built-in reference to the feedback or error signal.
A simple voltage detector can be made from a comparator and two inputs: one for the signal to be monitored, and the other for a voltage representing the trip point. This trip level can be set by a pair of resistors or by an internal/external voltage reference. A pair of resistors makes a simple reference, but the resulting voltage is at the mercy of the source driving the divider and of the stability of current drawn from the resistor-divider node. By contrast, a Maxim MAX917 voltage detector contains a comparator and an internal voltage reference.
Important voltage-reference specifications include, but aren't limited to:
- Initial accuracy
- Output-voltage temperature drift
- Current source-and-sink capability
- Quiescent current
- Long-term stability
- Output-voltage temperature hysteresis
- Noise
- Cost
The main types of voltage references are based on the zener diode, the buried-zener diode, and the bandgap device (Table 1). Each type can be configured as either a two-terminal shunt topology or as a three-terminal series topology. Zener diodes, or those diodes intended to operate in the reverse-bias region, require a series current-limiting resistor. Zeners typically aren't well suited for high-precision or low-power applications. The BZX84C2V7LT1 zener, for ex-ample, has a nominal VOUT of 2.7 V, but it varies from unit to unit, from 2.5 to 2.9 V, a tolerance of about ±7.5%.
An ideal voltage reference should have zero source impedance, letting it maintain a constant output voltage, regardless of the current it sinks or sources. Actual source impedance is never zero, but low levels in the milliohm range are possible. On the other hand, the BZX zener has an internal impedance of 100 Ω at 5 mA and 600 Ω at 1 mA. Nevertheless, zener diodes are very useful in voltage-clamping circuits. They can handle a wide range of clamp voltages (2 to 200 V) and a wide range of power, from several milliwatts to several watts.
The MAX6330, a shunt device with power-on-reset output, avoids some of the drawbacks of zeners (Fig. 1). It has a tight initial accuracy (within 1.5% or better) over the full IOUT range of 100 µA to 50 mA. As with all shunt devices, designers should consider the following factors when selecting a proper shunt resistor, RS:
- Input voltage range (VIN)
- Regulated voltage (VSHUNT)
- Output current range (ILOAD)
- Minimum shunt operating current (ISHUNT): always plan for the maximum load current plus ISHUNT
Designers should choose the highest nominal resistor value for RS that yields the lowest current consumption. The design safety has to accommodate the worst-case tolerance of the resistor used:
The following general power equation ensures an adequate power rating for the resistor:
A shunt topology always draws ILOADMAX + ISHUNT, whether or not a load is present. On the other hand, shunt references have an advantage. By properly sizing RS, the same shunt can operate from 10 to 100 VIN. Typical ISHUNT values are in the range of 10 to 60 µA.
The principle behind bandgap references is the summing of two VBE voltages. Because one voltage has a positive temperature coefficient (TC) and the other a negative TC, their sum at the output has a zero TC (Fig. 2). Of course, actual output TCs never equal exactly zero. IC design, packaging, and manufacturing-test capabilities all affect the output TC. With care, though, it's possible to attain reasonably low VOUT TCs between 5 and 100 ppm/°C.
The absence of an external resistor in a three-terminal bandgap or other series-mode voltage reference simplifies the design and minimizes power consumption. But it isn't possible to simply insert the reference on a board and forget about it. The surrounding circuitry might require special performance from the reference. A ±5% power supply and an 8-bit data-acquisition system, for instance, place much looser demands on the reference as opposed to a micropower system where current consumption must be as low as possible for each component. In such cases, a 2.5-V reference like the MAX6025 or MAX-6192 draws a supply current of only 35 µA maximum, which is virtually independent of IOUT.
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.