Selecting the Right Series Vol

Abstract: This arTIcle provides an in depth analysis, procedure, and selecTIon tables for selecTIng the opTImum series voltage reference for voltage output DACs. It covers all of the important parameters such as: input supply voltage, reference output voltage, initial accuracy, line and load regulation, stability, and noise. In addition four common design examples are given.

When designing a system that includes a digital-to-analogconverter (DAC) and an external voltage reference, the voltage-referencespecifications are as important as the specifications of the DAC itself.This article explores some of the issues involved with selecting external,three-terminal series voltage references for voltage-output DACs. Examplesof DAC system designs are used to illustrate the various trade-offs whenoptimizing for cost, accuracy, or power. A Few Words about Voltage ReferencesThe main focus of this article is Maxim's three-terminal, series, bandgap voltage references, although buried-zener references are discussed as well. Two-terminal, shunt references are not covered, because three-terminal, series devices are now available at competitive prices and with low quiescent current that is virtually constant versus input voltage.

The MAX6006-MAX6009 two-terminal shunt-reference family is worth considering for ultra-low-power applications, as they can generate 1.25V, 2.048V, 2.5V, or 3V with an operating current of only 1µA. Maxim also offers a family of low-cost, industry-standard LM4040 shunt references.

Zener-based references receive light treatment primarily because of their high input-voltage requirement (which limits their applicability in systems with lower supply voltages). In spite of this narrowing of focus, many topics covered in this article are applicable to other reference types. For example, the reference-voltage change on a shunt reference caused by varying bias currents is analogous to the load regulation of a series reference, and the effect on the DAC performance can be analyzed in a similar manner.

Figure 1 depicts the connection between a three-terminal, series voltage reference (the MAX6325) and a DAC (the MAX5170) for a typical design. In this case, an external capacitor is shown between the reference and the DAC, but it can usually be eliminated if space is limited and if the DAC does not have rapid power supply or output transient switching. The figure also shows a power-supply filtering input capacitor and a broadband noise-reduction capacitor, but both of these capacitors are also optional. Finally, the MAX6325 voltage reference has a TRIM pin that allows the end user to optionally trim out the initial error with an external potentiometer.


Figure 1. Interfacing a three-terminal series voltage reference and a DAC. Definitions of Voltage-Reference SpecificationsIn addition to cost and packaging, several (but not all) specifications will be covered in the discussions on voltage-reference selection. These specifications are described in detail in the references listed at the end of the article, so they will be only briefly summarized here. What follow are definitions of the specifications:

Input Supply Voltage: Power-supply input voltage to the reference. Limited on the upper end by the silicon process used for the reference and on the bottom end by the reference output and dropout voltages:

VIN(MIN) = VREF + VDROPOUT

Reference Output Voltage: Regulated voltage used at the DAC reference input.

Initial Accuracy: Accuracy is a slight misnomer, as it actually represents the initial output-voltage error. Specified in % or mV at 25°C. Some fixed-voltage and any adjustable references can be trimmed to improve accuracy.

Reference Output Current: The available load current that the voltage reference can source to the DAC reference input. All of the references covered in this article can also sink current, but not as well.

Reference-Load Regulation: Incremental change in reference output voltage for a DC change in reference output current. Specified in µV/µA or equivalently mV/mA, mV (over the entire output-current range), ppm/mA, or %/mA.

Input-Line Regulation: Incremental change in reference output voltage for a DC change in input supply voltage. Specified in µV/V.

Output-Voltage Temperature Coefficient (Tempco): Change in reference output voltage for a given change in temperature. Specified in ppm/°C. Maxim uses the box method, where the maximum reference output fractional voltage change is divided by the maximum operating-temperature range:

TCVOUT = 106 ×| ΔVREF(max)/ VREF | / (TMAX-TMIN)

Output-Voltage Temperature Hysteresis: Change in reference output voltage at +25°C after a temperature cycle (TMIN to TMAX) is applied. Specified as a ratio of voltages expressed in ppm.

TEMPHYST = 106 × | ΔVREF / VREF |, where ΔVREF = VREF before ΔT cycle minus VREF after ΔT cycle.

Output-Voltage Long-Term Stability: Change in reference output voltage versus time. Specified in ppm/1000 hours. Cumulative drift beyond a 1000-hour interval is not specified, but it is usually much lower than the initial drift, which can itself be improved by PCB-level burn-in.

Output Noise Voltage: Voltage noise at the reference output. The 1/f component is specified in µVp-p over a 0.1Hz to 10Hz bandwidth, and the wideband noise is usually specified in µVRMS over a 10Hz to 10kHz bandwidth.

Capacitive-Load Stability Range: Range of capacitive loads (includes user-supplied capacitors, the capacitance presented by the load, and stray capacitance) that the reference can tolerate at its output. External capacitors are needed only to limit large loads or supply transients, and can be eliminated in many designs to save board space. A few references have pins where a compensation capacitor (the MAX872) or a noise-reduction capacitor (the MAX6325) can be added to improve performance. DAC ConsiderationsOnly buffered-voltage-output DACs are discussed, as the key points are easier to illustrate with this architecture. Current-output DACs are typically used in a multiplying configuration (MDAC) to provide variable gain, and they usually require external op amps to generate a voltage output.

All of the Maxim voltage-output DACs considered in this article use an inverted R-2R architecture. From the reference-voltage standpoint, the main characteristic of this DAC architecture is the varying DAC reference input resistance versus DAC code. Care must be taken to ensure that the voltage reference can source enough current at the DAC's minimum reference input resistance and has sufficient load regulation as the DAC codes change. A 4-bit example, along with its normalized reference input current, is shown in Figure 2. Note that the reference current at DAC code 0 is not shown in the plot, as all of the switches connected to the reference are open and virtually no reference current flows. Two other DAC specifications important to voltage-reference selection are reference-input-voltage range and DAC output gain.


Figure 2. Inverted R-2R architecture and reference-input-current variation (4-bit). Output Error and Accuracy DefinitionsWe define output error as the deviation from an ideal output voltage that would be provided by a perfect voltage reference and DAC. It's important to note that we are addressing absolute accuracy in this article, meaning that everything is referenced to an ideal DAC output-voltage range. For example, a 12-bit DAC code 4095 should produce an output of 4.095V with a reference voltage of 4.096V; any deviation from this is an error. This is in contrast to relative accuracy, where the full-scale output is defined more by the application than by an absolute voltage. For example, in a ratiometric system where an ADC and a DAC with equal resolution share a reference, it may not matter (within reason) what the actual reference voltage is, as long as the DAC-output and ADC-input voltages are nearly equivalent for a given digital code.

Output error is often specified as a one-sided value (in LSBs at the DAC resolution), but it actually implies a double-sided error (Figure 3). For example, a 12-bit DAC with a 4.096V output range has an ideal LSB step size of 4.096V/4096 = 1mV. If the specified output error in this case is 4LSBs at 12-bit resolution, this means the DAC output at any code could be ±4LSBs (or ±4mV) from the ideal value. We define accuracy in terms of how many actual bits we have at our disposal to reach a desired output voltage with at most 1LSB of error:

Accuracy = DAC Resolution - log2(Error)

In our example, we effectively have 10 bits (12 - log2(4)) of accuracy, because we can only get to within 1LSB at 10-bit resolution (±4mV = ±4/4096 = ±1/1024) of any given ideal DAC output value.


Figure 3. DAC transfer function and output error (zero offset assumed).

There are several sources that contribute to output error, but some (such as DAC offset) are ignored because they are not relevant to the reference-voltage selection process. Voltage-reference error sources that are considered include initial error, tempco, temperature hysteresis, long-term stability, load and line regulation, and output noise. DAC error sources include INL, gain error, gain-error tempco, and output noise.

Although the target error applies over the entire DAC code range, most of the error sources mentioned above cause an effective gain-error variation that is largest near the full-scale (highest DAC codes) of the transfer function (Figure 3). Gain errors are reduced with decreasing DAC code value; these errors are halved at midscale, and they virtually disappear near code zero, where offset error dominates. Error sources that do not exclusively affect the gain error and apply equally over most of the DAC code range include DAC INL and output noise.

INL is typically defined using one of two methods: absolute linearity or end-point linearity. Absolute linearity compares the DAC linearity to the ideal transfer-function linearity. End-point linearity uses the two measured end points to define the linearity (a straight line is drawn between these points), and all other points are compared to this line. In either case, INL should be included in the error analysis. In the latter case, the DAC INL error is zero at the end points, but can be present at DAC code words just inside these values. As an example, for a 12-bit DAC with INL defined between the end points of 0V and 4.095V (full-scale), the INL specification applies to DAC codes near 0 and 4095. For maximum error calculations, it is reasonable to add the DAC's INL and noise-induced output errors to the previously mentioned gain errors that are most severe near code 4095. DAC Design ExamplesTo illustrate the steps involved with voltage-reference selection for DACs, a few design examples were created to cover a range of applications (Table 1). Only 10-, 12-, and 14-bit DAC designs are included in these examples, because they are the most instructive. The design steps are broken into individual sections by design examples (see Design A, Design B, and so forth).

Table 1. Requirements for DAC Design Examples
Parameter Design A Design B Design C Design D Main Design Objectives