电压引用如何在A/D和D/A转换中

Article By : Don Dingee

为什么在混合信号设计中需要电压参考?万博投注网址在处理数字电源转换时,它归结为准确性和稳定性。

上次,在我的使用集成A / D转换器在微观controller or system-on-chip (SoC), I said: “An external voltage reference pin may allow a higher voltage source (compared to the digital power rail for the microcontroller itself) for a wider analog input range, or a more stable source for higher accuracy.”

That was a tad oversimplification; so, how voltage references factor into conversion is worth a second look.

电源不是过去的

In the before-maker times, analog circuitry often ran on ±12 VDC或±15 VDCpower, while digital circuits usually ran on +5 VDC。Staying away from “the rails”, analog signals were usually kept in the ±10 VDC范围,使每米洛伏型单位量表变得容易数学。通过线性调节,供应电压通常非常干净。噪声仍在背景中,但是大多数时候较大的信噪比(SNR)覆盖了它。A/D和D/A转换器位分辨率很小,因此步长也比噪声大得多。

Then digital-first thinking took over. Shrinking digital semiconductor processes made +5 V operation mostly obsolete—with one notable exception we’ll see shortly—with stops at +3.3 V, +1.8 V, +1.2 V, and even sub-volt operation. Those clean but wildly inefficient linear-regulated power supplies gave way to more efficient but noisier switched-mode power supply technology. Mixed-signal processes brought analog circuitry on digital chips, often running at lower digital power supply voltages. Converter resolutions increased, narrowing step widths. All this worked against analog signal-to-noise ratio, raising concerns for converter accuracy.

可能有别针

Back to my opening statement—of course, the operative word is “may.” Lower-end parts often provide only an internal reference voltage for the A/D or D/A converter. This is especially common in a low pin count package, with no room for an external voltage reference pin. Small packages tend to go on small boards. An internal voltage reference saves external parts and takes up less space. If that’s what there is to work with, work with it.

For bigger parts, there is often a choice between an internal reference voltage, and one supplied on a V参考别针。使用内部引用的弊端之一是,它可能以零为零的范围抵消了其值。这使转换器的中点位于该范围的中间。基于外部操作AMP的水平转移电路可能会居中以更好的优势。

许多制造商已经想到了V参考将转换器范围排列到±V参考和零电压的中点。即使只是将范围加倍的范围都可以提高信噪比。另外,控制V参考可能更友好。而不是使用2.5 V或3.3 V之类的东西,而是V参考might be set to a binary value like 4.096 V, an exact figure matched to a 12-bit converter with 4096 values.

这是Precision LM4040电压参考突破的样子。资源:Adafruit

Steal some volts and add stability

One might ask: if a part is running at say, 3.3 VDC,如何获得V的更高电压参考?好消息是V参考does not need a lot of current behind it. There may be a higher voltage running around on a board. For instance, if a board has a USB port, there’s 5 VDCavailable close by. Small voltage reference parts can transform that into a lower V参考。如果需要开关调节器,则需要一个小零件,需要外部电感器和其他一些零件,以创建更高的电压以撞到电压参考。

那么,为什么需要电压参考零件?这取决于准确性和稳定性。请记住,在某些情况下,数字电源可能会有所不同。使用数字功率进行转换器将其直接从中倒入转换结果中。电压参考受到更严格的控制。初始准确性值通常在0.05%至0.5%范围内。噪声通常以数百微伏或更少的方式测量。温度系数也保持较低;板并不总是在室温和V处运行参考可以漂移。

外部v参考并且创建它所需的零件可以帮助转换器的准确性,并轻松使用结果。有良好的选择指南,有关电压参考如何从网上从几家制造商转化为转换的更多选择指南。这是一个高级主题,但是如果您花更多的时间用于更准确的A/D或D/A转换器,则投资外部参考是有意义的。

本文最初发表在Planet Analog

在一般动态的导弹指导系统中花费了十年之后,Don Dingee成为摩托罗拉的Vmebus和单板计算机技术的传教士。他写了有关行星类似物的传感器,ADC/DAC和信号处理。

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