How to use voltage references to ensure accurate, stable data conversion

06/22/2019

In order to connect the analog and digital worlds more quickly and efficiently to take advantage of the Internet of Things (IoT), it is easy but unwise to ignore the key role of voltage references. Analog-to-digital (ADC) and digital-to-analog (DAC) converters are used as the primary criteria for “judgement” of analog input and output values, helping to ensure accurate signal and data conversion, but only for proper selection and proper application.


This article will briefly introduce the structure and characteristics of the voltage reference and describe how to select the voltage reference. For example, it will introduce ADI's ADR43x family of voltage references to illustrate the various features available to designers, enhancements and features to take advantage of modern voltage references. In the process, it will show how to apply the ADR43x device to within acceptable limits so that the ADC, DAC, and the entire system can realize their full potential.


The key role of voltage reference

In the basic form, the voltage reference is a three-terminal device with power rail, ground (common) and precision output voltage connections (Figure 1). References that are not suitable for this task or that are not applied correctly will be inaccurate and will compromise the validity and confidence of the converter output.


ADI LT6656 Series LT6656AIS6-2.5 Device Diagram

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Figure 1: The basic implementation of the voltage reference is a three-terminal device with input voltage, output reference, and ground (common) connections. The LT6656AIS6-2.5 device from the Analog Devices LT6656 family is shown here. (Source: Analog Devices)


Once the designer has chosen the appropriate reference for the nominal output voltage, accuracy and tolerance, and other parameters, the challenge is to use the reference so that the specified performance fully meets the application requirements and the performance of the device is uncompromising. The importance of this point cannot be underestimated. As mentioned above, the voltage reference is the primary criterion by which the ADC determines the analog input voltage when digitizing the voltage. In the case of a DAC, a stable and reliable voltage reference allows the converter to generate an accurate analog output voltage corresponding to the input digital code.


Select reference

Three techniques are most commonly used for solid-state references: buried Zener diodes, using the transistor V-bandgap method, works with Analog Devices' XFET® configuration, which has two junction field effect transistors in series (US Patent No. 5838192).


While voltage reference designers may discuss the nuances and attributes of each method (for good reason), for most voltage reference users, the focus is on performance, trade-offs, applications, and cost issues. This is the point of view taken here.


Due to the physical characteristics of the underlying devices used, the internal core reference of the voltage reference may be at a "clumsy" value, while the voltage reference design has internal circuitry to ensure that its output voltage matches the converter resolution well. And system requirements.


For example, many references are provided as a series of identical devices with selectable output values such as 2.048, 2.5, 3.0, 4.096 and 5.0 volts. The 2.048 volt and 4.096 volt versions are convenient because they are "uniformly" mapped to the converter resolution; for example, a nominal ratio of a 12-bit converter using a 4.096 volt reference is 1 millivolt (mV) per conversion count.


The initial reference accuracy is specified in percent or millivolts and the accuracy can vary greatly, as some applications require higher precision than other applications. Generally, higher accuracy is more difficult to implement and maintain; a typical reference specification is a maximum error of ±0.1% under all conditions. However, advances in the underlying topology and process technology have led to improvements in the specification. For example, the 4.096-volt ADR434 voltage reference uses the XFET method with an initial accuracy of ±5 mV (A suffix) or ±1.5 mV (B suffix).


However, the absolute accuracy of many applications is a secondary factor in reference stability and long-term consistency. The reason may be that the digitized data can be corrected later, or the absolute accuracy is not as important as the comparison result and its changes, both of which are functions of reference stability. Therefore, the choice of reference must assess how much absolute precision is needed and how much stability is needed and how to maintain stability.


This stability factor has important considerations. Is it for short-term use, such as when data is acquired during a short experiment? Or is it long-term data collection for more than a year or more? These are the questions designers must answer in advance for each project.


External reference and internal reference

There is a more basic question: Do you even need a separate external reference? Converters such as the Analog Devices AD7605-4BSTZ ADC have an internal reference voltage that saves board space and bill of materials (BOM) (Figure 2). In addition, the data sheet provides a specification of fully characterized ADC read accuracy because the performance of the reference becomes part of the overall performance of the converter IC.


Analog Devices 16-bit AD7605-4BSTZ Schematic (click to enlarge)

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Figure 2: Many ADCs (such as the 16-bit AD7605-4BSTZ) have an internal reference voltage. In addition to saving space and reducing BOM, this simplifies error budget analysis because reference performance is included in the overall specification of the converter. (Source: Analog Devices)


However, even if the converter core is suitable, the internal reference may not provide the required performance, so most converters have an external reference connection. Note that highly dedicated and cost-sensitive converters (such as low-end audio channel converters) may have internal converters that meet the target criteria, so no external reference is required. Nonetheless, it is simple to assume that any external reference automatically provides better results than the internal reference because the performance of the internal reference may be comparable to the specifications of its associated converter.


There is another reason to consider using an external reference voltage, even if the internal reference voltage is sufficient. In designs where there is not only one converter IC, the individual internal references may be different or different from each other. Due to differences in references, there will be inconsistencies in their resulting data, which makes it difficult to correlate data with unsolvable unsolvable errors.


Therefore, for high performance systems with multiple converters, it is often best to use a single shared external reference. However, doing so raises concerns about the ability to "drive" multiple converters without reducing their underlying performance, which is a consideration discussed below.


Maintenance reference performance

In addition to the initial accuracy and tolerance specifications, the references must address some issues to ensure that performance remains within acceptable limits. These issues include:


Layout issues, including voltage drop and noise

Output driver (source/receiver), load buffering and transient performance

Short-term stability and temperature-dependent drift

Long-term drift due to aging, physical stress and packaging

1. Layout issues, including voltage drop and noise: As with any sensitive analog signal, even a signal that provides a quiescent voltage, there may be an excessive current impedance (IR) voltage drop between the reference output and the converter. Although most reference loads are as low as tens of milliamps (mA), even a moderate load of 10 mA through 100 milliohms (mΩ) can cause a voltage drop of 1 mV, which can cause large errors in the budget. .


The ADR43x family of voltage references overcome this problem by including the wiring resistance in the forced loop of an external op amp (op amp) in a Kelvin connection configuration (Figure 3). The amplifier detects the load voltage, so the loop control of the op amp forces the output to compensate for wiring errors, producing the correct voltage at the load.


ADI's ADR43x diagram

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Figure 3: Devices in the ADR43x family can be configured as Kelvin connections through an external op amp so that any IR drop between the reference output and the converter reference input connections is part of the feedback loop and then the loss is corrected. (Source: Analog Devices)


Due to load noise, ground (common) noise, and noise pickup from insufficiently decoupled power rails, external noise can also affect the reference voltage on the converter. In addition, the reference also has low frequency (0.1 Hz to 10.0 Hz) and high frequency (10 Hz to 25 kHz) internal noise that must be evaluated. High-performance reference sources, such as the reference in the ADR43x family, have low-frequency noise below 3.5 microvolts (μV) peak-to-peak (pp) and high-frequency noise.


The noise density spectrum of ADR431BRZ-REEL7 is shown (Figure 4). For different capacitive loads, it is relatively flat to approximately 1 kHz and then begins to rise; for a zero capacitive load, it remains flat.


Relationship between noise density and frequency of ADR431BRZ-REEL7

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Figure 4: The noise density of the ADR431BRZ-REEL7 is relatively flat to about 1 kHz with different capacitive loads and then begins to rise; it remains flat for zero capacitive loads and increases faster as the load increases. (Source: Analog Devices)


The most common strategy for reducing noise is to add a simple resistor-capacitor (RC) filter. However, the output amplifiers of many references may become unstable and oscillate under large capacitive loads, so unless a reference design is used for the output, it is not possible to choose to connect a larger capacitance of a few microfarads (μF) to the output. For ADR43x devices, if the high frequency noise is still out of specification, a simple RC filter can be used to supplement the basic connection of the reference (Figure 5).


Basic connection diagram of ADR43x reference

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Figure 5: The basic connection of the ADR43x reference requires only a few passive external components, two capacitors on the input side and a basic 0.1μF capacitor on the output. (Source: Analog Devices)


Note that the ADR43x provides an external pin for each pin that provides access to the internal compensation node, allowing the addition of an external series RC network at critical circuit points (Figure 6).


ADI's ADR43x device diagram provides user-accessible package leads

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Figure 6: The ADR43x device has a user-accessible package pin (pin 7) that can be used to add the required compensation to the internal op amp. (Source: Analog Devices)


Adding an RC circuit allows the user to "overcompensate" the internal op amp and avoid instability. The user can select the capacitance value to achieve an acceptable low noise level versus frequency (Figure 7).


ADR43x noise reduction map

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Figure 7: Designers using the ADR43x reference can select RC component values to achieve the desired level of noise reduction without regard to output instability, as shown by the noise density vs. frequency plot for various RC combinations. (Source: Analog Devices)


2. Output Driver (Source/Receiver), Load Buffering and Transient Performance: Most voltage references are internally buffered to provide and sink up to 5 or 10 mA. An external buffer (usually unity gain) is required if the required load current is greater than the reference source/absorption rating. However, buffers may not be desirable because the potential effects of defects (inaccuracy, drift) may cause the reference to exceed system specifications.


In many cases, the ADR43x family does not require an external current boost buffer because it has a relatively high +30 mA supply and a -20 mA sink current rating.


In addition, the load on the reference is not necessarily constant, but may vary as the ADC (or DAC) switches internally. This is not a problem if the external reference input on the converter is buffered; if not, the transient performance of the reference must be checked. In some cases, an external buffer is required between the reference and the converter to provide the drive under transient load; again, the performance of the buffer must be considered in the system error analysis.


3. Short-term stability and temperature-dependent drift: The reference output will drift due to the stable time of the active circuit and the thermal gradient on the chip. The turn-on settling time of most reference sources is usually dependent on the load capacitance, but the load capacitance has the least impact on the ADR431 with less load (Figures 8 and 9).


ADR431 conduction establishment time chart

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Figure 8: The turn-on settling time of the unloaded ADR431 is approximately 8 microseconds (μs). (Source: Analog Devices)


ADR431's turn-on time diagram, increasing the load of 0.01μF

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Figure 9: By adding a 0.01μF load, the turn-on settling time of the ADR431 is only about 8μs. (Source: Analog Devices)


The data sheet specifies the reference accuracy at the defined temperature, usually different from the open value. Output changes due to temperature changes can easily exceed system accuracy requirements, so references with appropriate low drift specifications are required. The ADR43x series is rated for operation from -40°C to +125°C; for the ADR434A (4.096 volts, ±5 mV initial accuracy), the factor is 10 parts per million (ppm)/°C, while the series Other members have values as low as 3 ppm / °C.


4. Long-term drift due to aging, physical stress and packaging: Drift is often an important reason for inaccurate references. Consider an application that requires a voltage reference with a total accuracy of ±0.1% over the temperature range. Designers can choose a high performance reference with an initial accuracy of ±0.05% and a very low temperature coefficient of ±5 ppm / °C.


Between 25 ° C and 125 ° C, the drift caused by the temperature coefficient will be 5 ppm / ° C × 100 ° C, or 500 ppm (0.05%), so the total error (initial error + drift error) will meet the requirements ±0.1%. Some high-end applications place the reference in a temperature-controlled oven, similar to an oven for temperature-stabilized frequency setting crystals and clocks, but this is undesirable or impractical for most situations.


As the reference accuracy increases, its fundamental long-term drift (LTD) becomes a larger factor in maintaining this accuracy. For design engineers, LTD presents a special challenge because it is also a function of the production process and product usage patterns, not just the thoroughness of the design and the associated component selection. The stress on the package that occurs during board assembly is the main reason for LTD. Due to the high temperature exposed to the board soldering process, the plastic packaged IC will change shape slightly, and the dimensional changes caused by this stress will put pressure on the voltage reference chip.


The result is that the output of the voltage reference changes with these mechanical, assembly-related stresses that decay and return to normal within hours, days, or even weeks. The amount of variation depends on the layout, device package and other factors, and is typically on the order of tens of ppm. In addition, when the device ages within a year, the reference chip and package relationship is even "stable", so some references specify drift over a longer period of time.


Most reference data sheets provide the LTD specification as a typical drift 1000 hours after the run; the ADR43x series data sheet specifies a 40 hour (typical) 1000 hour LTD, but also notices that the drift over the next 1000 hours is significantly lower than the previous 1000 Drift within hours.


One solution to this stress-induced drift is to cycle the plate several times in a few hours as this will accelerate the relief of internal stress. Another solution is to consider the use of voltage references in ceramic packages because they are generally more stable than plastic packages and have lower bending levels than plastic packages. However, there are not many references in ceramic packages; this may not be a problem, as the latest generation of plastic references offer LTD performance that is almost as good as ceramic packaged devices.


Finally, designers can't ignore the effects of transient voltage references on their own power rails; after all, the reference is a specialized "power supply" in many ways. Therefore, load variations can not only affect output accuracy, but a stable and clean direct current (DC) input line is another factor that maintains specified performance. In other words, a well-designed voltage reference will strictly adjust the power input. The ADR 431 specifies the profile ΔV OUT / ΔV IN in the input voltage range of 7 to 18 volts (Fig. 10) of 5 millivolts/ppm (typical) and 20 millivolts/ppm (maximum).


Although the line transient voltage is 500 mV, the output of the ADR43x device chart does not change.

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Figure 10: Transient voltage in the voltage reference rail can adversely affect its performance, but good internal line regulation should solve this problem. For example, although the line transient voltage is 500 mV, the output of the ADR43x device does not change. (Source: Analog Devices)


in conclusion

Whether it is an internal or discrete external component of an ADC or DAC, the voltage reference is a key component of any system that uses a data converter. Improvements in basic accuracy, drift, and other parameters translate into system-level performance improvements.


As shown, designers can provide a variety of voltage reference features and enhancements in topology and process. In addition to the added functionality to ensure accuracy and consistent performance under a variety of static and dynamic operating conditions, seemingly simple voltage references have many designers looking for options that meet stringent design requirements.