So how do I choose a voltage reference?
I seem to have opened a can of worms with my articles on how to choose components for analog circuitry. When the Sun cools1, I fear that I may still be writing ‘em.
Read the RAQ on choosing analog ICs2 and its associated longer article3 to learn about general issues involved in choosing an analog IC. Each type has particular parameters that must be optimized, though, and here I shall discuss voltage references—devices that produce a stable, accurate dc voltage that defines the precision of ADCs, DACs, and other analog circuits.
A voltage reference is intended to produce an accurate voltage, so the value and the precision of the output voltage are obviously very important. In addition, be sure to consider device-specific parameters such as temperature drift, long-term stability, output circuitry, headroom, and noise.
A limited range of output voltages is available, almost all are between +0.5 V and +10 V. As far as I know, no three-terminal negative references are commercially available4, but two-terminal (shunt) references may be used with positive or negative supplies. In addition to fixed-voltage references, some references allow output programming with one or two external resistors. The accuracy and stability of these references are, of course, affected by the accuracy and stability of the resistors as well as by the reference’s own internal accuracy.
So what accuracy and stability can we hope for? The AD588 specifies 0.01% max initial error (one part in 10,000 or about 13 bits), with a 1.5-ppm/°C max temperature coefficient. Over the industrial temperature range of –40°C to +100°C, this could cause a 210 ppm variation, or 1 LSB at 12 bits. So, without temperature compensation, the best uncalibrated absolute accuracy we can guarantee is about 12 bits over temperature5. If we calibrate using expensive high-precision voltage standards (racks of equipment, not ICs), and limit the temperature range that the IC sees to ±20°C around room temperature, we might just achieve a temperature-compensated absolute accuracy of about 16 bits.
If the temperature varies over a large range, however, thermo-mechanical hysteresis will limit a voltage reference’s repeatability to about 14 bits6 no matter how well they are calibrated and temperature compensated.
The data sheets of many references specify long-term drift—typically about 25 ppm/1000 hr. This error is proportional to the square root of elapsed time, so 25 ppm/1000 hr ≈ 75 ppm/year. The actual rate is likely (but not certain) to be somewhat better than this as the ageing rate often diminishes after the first few thousand hours. So, again, we have a figure around 14 bits.
The two basic types of reference output architecture are series and shunt. A shunt reference resembles a Zener diode in that it has two terminals and sinks variable currents at a fixed voltage. A series regulator has three terminals—input, output, and ground7. A dc voltage greater than the reference voltage is applied to the input, and the output provides an accurate reference voltage. Most references require the input voltage to be a volt or more above the output, but low-dropout references allow a difference as small as a few tens or hundreds of millivolts.
The simplest series voltage references have emitter follower output stages and can only source current, but many reference applications require the reference to sink current as well. This must be checked when an application requires current to flow in both directions.
The mechanisms used to generate precision reference voltages can be somewhat noisy, so it is important to verify that the reference noise is low enough for your application. Mid-band noise (above 100 Hz) can have spectral density of tens of mV/√Hz or more, but can usually be filtered with a capacitor, provided that the reference is stable with capacitive loads. Note that even if the reference is stable, a capacitive load may increase its turn-on time. Low-frequency noise, which may be more troublesome, is usually specified in a low-frequency band, often 0.1 Hz to 10 Hz. Less than 5 μV pk‑pk is good; 1 μV pk-pk to 2 μV pk-pk is exceptional.
Other considerations that apply to general analog ICs also apply to voltage references.
1Do not worry about this. When the Sun finally gutters out it is confidently estimated that commercial fusion power will by then be no more than thirty years in the future.
4A few negative series regulators exist, but they are not really precise enough to use as voltage references. There is no technical reason why negative three-terminal references should not be designed, their absence seems to be due to lack of demand. (“You're the fiftieth person I've told today: we don't sell them, there's no demand!”)
7This is in the basic device; other terminals, such as chip enable, noise filtering, and sense and force output terminals may also be present in more complex references.