Section 7.1: Voltage References

Reference circuits and linear regulators actually have much in common. In fact, the latter could be functionally described as a reference circuit, but with greater current (or power) output. Accordingly, almost all of the specifications of the two circuit types have great commonality (even though the performance of references is usually tighter with regard to drift, accuracy, etc.). This section discusses voltage references, and the next section covers linear regulators, with emphasis on their low dropout operation for highest power efficiency. Voltage references have a major impact on the performance and accuracy of analog systems. A ±5-mV tolerance on a 5-V reference corresponds to ±0.1% absolute accuracy—only 10 bits. For a 12-bit system, choosing a reference that has a ±1-mV tolerance may be far more cost effective than performing manual calibration, while both high initial accuracy and calibration will be necessary in a system making absolute 16-bit measurements. Note that many systems make relative measurements rather than absolute ones, and in such cases the absolute accuracy of the reference is not important, although noise and short-term stability may be. Figure 7.1 summarizes some key points of the reference selection process. Temperature drift or drift due to aging may be an even greater problem than absolute accuracy. The initial error can always be trimmed, but compensating for drift is difficult. Where possible, references should be chosen for temperature coefficient and aging characteristics which preserve adequate accuracy over the operating temperature range and expected lifetime of the system. Noise in voltage references is often overlooked, but it can be very important in system design. It is generally specified on data sheets, but system designers frequently ignore the specification and assume that voltage references do not contribute to system noise. There are two dynamic issues that must be considered with voltage references: their behavior at start-up, and their behavior with transient loads. With regard to the first, always bear in mind that voltage references do not power up instantly (this is true of references inside ADCs and DACs as well as discrete designs). Thus it is rarely possible to turn on an ADC and reference, whether internal or external, make a reading, and turn off again within a few microseconds, however attractive such a procedure might be in terms of energy saving. Regarding the second point, a given reference IC may or may not be well suited for pulse-loading conditions, dependent upon the …