To achieve high-precision data acquisition and conversion, the reference voltage source is critical

【Introduction】Digital signal processing provides a cost-effective way to correctly allocate information, such as bandwidth or capacity. Consider a very basic digital transceiver design consisting of a receive path and a transmit path. In the receive path, a continuous analog signal representing some information element is captured at a definite point in time. This signal can be represented as a time-varying signal. Voltage swing, the conversion step is of course also converting the voltage swing into a digital representation, and the resulting digital representation or code can be processed to extract the information content in the signal. Certain judgments can be made based on the characteristics of the information, such as routing the information to another network node, or doing some kind of operation on the transceiver at a specific time, sending the exact opposite path, the information needs to be delivered to the actual device. A digital-to-analog converter produces an analog output that corresponds to the relative value of a digital input signal relative to a fixed reference value, in its most basic form determined by the relationship shown in the figure below.

To achieve high-precision data acquisition and conversion, the reference voltage source is critical

A key element of the conversion from the digital to the analog domain is a series of finite discrete values, now represented by an analog variable, which leads to quantitative uncertainty. A reference quantity, either voltage or current, is precisely divided into some binary and/or linear segments, digital inputs drive switches to connect the appropriate number of segments to the output, digital inputs can be provided in different forms, such as transistor-to-transistor logic ( TTL), Complementary Metal Oxide semiconductor (CMOS), or Low Voltage Differential Signaling (LVDS).

DACs often require ancillary products such as a voltage reference and buffers for the reference input pins and the DAC output. In precision DACs, the reference performance is critical to the overall DAC performance because errors in the reference are reflected in the output of the DAC. In any mixed-signal system, the voltage reference is the most important part because any change in it affects all other parts in the system. Some references can drift if the reference current is not stable, and changes in the DAC reference current can affect the reference voltage.

The main specifications to consider when choosing a voltage reference are: absolute error in noise temperature drift (although this can easily be removed by calibration) and long-term stability, techniques such as paralleling multiple references can be used to reduce noise and time drift . DACs often contain an on-chip reference and/or an on-chip reference buffer. If the DAC does not have an on-chip reference or on-chip reference buffer, it may be necessary to buffer the input reference pins. The DAC data sheet provides the input Impedance specification so that the user can calculate. Whether the reference can supply sufficient current when using this value is complicated by the fact that the input impedance of some DAC architectures, such as voltage-mode R-2R DACs, varies with the digital code applied to the DAC. There is a large variation, in which case the external reference voltage needs to be buffered, the reference voltage buffer of the DAC should be a low noise, low offset error amplifier, because the offset error of the reference voltage buffer will become the gain error at the output of the DAC .

As mentioned earlier, the reference buffer for the voltage DAC should be a low noise, low bias error amplifier, because the bias error of the reference buffer becomes the gain error at the output of the DAC. When choosing a buffer at the output of the DAC, you can Optimizing an amplifier for an application requires several considerations, faster settling time or higher bandwidth? Or require higher accuracy and lower noise? Cost, packaging, size, and channel count are also considered. Generally speaking, the settling speed of DAC switches is fast, so the slew rate and settling time of the DAC circuit are mainly determined by the output amplifier, and the output buffer generally requires low bias current, low bias error and sufficient headroom.

In applications where accuracy is a key requirement, the output buffer also requires low noise, while in applications requiring higher speed, an op amp with faster settling time, faster slew rate, and higher bandwidth should be selected. Ultimately, which amplifier to choose depends on the application.

Consider another example of a product that includes on-chip buffering. This is the block diagram of AD5754R series quad-channel DAC of Analog Devices (ADI), which has both on-chip reference voltage buffering and on-chip output buffering. The internal 2.5V reference voltage is buffered on-chip, so no external buffering is required. The output buffer shown here comes with a full-scale adjustable circuit that provides multiple user-configurable ranges, including unipolar and bipolar. The AD5754R is an easy-to-use, ready-to-use single-chip solution for building systems that eliminates many of the supporting circuits, including buffering and gain components required for these implementations, so it is a complete single-package solution that provides Predictable rating accuracy and performance to reduce system design time and simplify PCB layout.

Summary of this article:

Voltage references come in many forms and offer different characteristics, but at the end of the day, accuracy and stability are the most important characteristics of a voltage reference because its primary role is to provide a known output voltage. Changes from this known value are errors, and reference specifications typically use the following definitions to predict their uncertainty under certain conditions: initial accuracy, temperature drift, long-term stability.

Voltage references are typically used for accurate reference voltage settings for ADCs, DACs, and other analog circuits, as well as for biasing sensors, powering or driving components/systems, and virtual ground settings. A voltage reference is simply a circuit or circuit element that produces a stable, accurate DC voltage. If a product needs to capture relevant real-world information, such as battery voltage or current, power consumption, signal magnitude or characteristics, fault identification, etc., the relevant signal must be compared to a standard.

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