什麼是偽差分輸入(Pseudodifferential input)?

偽差分信號連接方式減小了噪聲,並允許在儀器放大器的共模電壓範圍內與浮動信號連接。

在偽差分模式下,信號與輸入的正端連接,信號的參考地與輸入的負端連接。偽差分輸入減小了信號源與設備的參考地電位(地環流)不同所造成的影響,這提高了測量的精度。

偽差分輸入與差分輸入在減小地環流和噪聲方面是非常相似的,不同的方面在於,差分輸入模式下,負端輸入是隨時間變化的,而在偽差分模式下,負端輸入一定僅僅是一個參考。描述偽差分的另外一種方式就是,輸入僅僅在打破地的環流這個意義上是差分的,而參考信號(負端輸入)不是作為傳遞信號的,而僅僅是為信號(正端輸入)提供一個直流參考點。


<span =“apple-style-span” style=“font-size: large;”>Differential Inputs

<span =“apple-style-span” style=“font-size: large;”>Both differential and pseudodifferential inputs provide common-mode voltage rejection while single-ended inputs do not. However, differential inputs provide both AC and DC common-mode rejection and pseudo-differential inputs provide only DC common-mode voltage rejection. For sensors requiring excitation, including microphones and accelerometers powered with IEPE, pseudo-differential inputs must be used to zero out the bias voltages and allow the sensor ground to be different than the measurement device ground. A diagram showing the differential input configuration of the NI 446x is shown below with a ground-referenced sensor.

<span =“apple-style-span” style=“font-size: large;”>差分和偽輸入提供共模電壓抑制,而單端輸入沒有。然而,差分輸入提供交流和直流共模抑制和偽差分輸入只提供直流共模電壓抑制。對於傳感器需要激勵,包括供電的麥克風和加速度計的IEPE,偽差分輸入必須用於零出偏置電壓,使傳感器的理由以不同於地面的測量裝置。阿圖顯示差分輸入配置的NI 446x如下以地為參考的傳感器。

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<span =“apple-style-span” style=“font-size: large;”>In theory, a differential input should be able to reject all DC common-mode voltages. However, most measurement devices specify a common-mode range. This is the input range over which a circuit can handle a common-mode signal. The common-mode signal is measured as the mathematical average voltage, relative to the measurement device’s ground, of the signals from a differential input. If this range is exceeded, users may damage the device. Pseudodifferential inputs are also subject to a common-mode range.

<span =“apple-style-span” style=“font-size: large;”>從理論上講,一個差分輸入應該可以拒絕所有的直流共模電壓。然而,大多數測量設備指定一個共模範圍。這是輸入射程超過該電路可處理的共模信號。共模信號計量的數學平均電壓,相對於測量設備的地面,從一對差分輸入信號。如果超出這個範圍,用戶可能會損壞設備。偽投入也受到了共模範圍。

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<span =“apple-style-span” style=“font-size: large;”>Pseudodifferential Inputs

<span =“apple-style-span” style=“font-size: large;”>A pseudodifferential input is much like a differential input in that it provides common-mode voltage rejection (unlike single-ended inputs). However, pseudodifferential inputs are all referred, but not directly tied, to a common ground. This connection is typically made by a relatively low value resistor to give some isolation between the sensor/signal ground and the measurement device ground. DSA measurement devices have a 50 ohm resistance between the negative signal connector, often the outer shell of a BNC, and the device ground. A diagram showing the pseudodifferential input configuration of the NI 446x is shown below with a floating sensor.

<span =“apple-style-span” style=“font-size: large;”>輸入是一個偽差分輸入很像,因為它提供了共模電壓抑制(與單端輸入)。然而,偽投入都提到,但不是直接關係到一個共同點。這種連接通常是由一個相對較低的電阻值,使一些孤立之間的傳感器 /信號的地面和地面測量裝置。數字減影血管造影測量設備有50歐姆的電阻連接器之間的負信號,通常是一個廠家,外殼和地面設備。阿圖顯示了偽輸入配置的NI 446x如下浮傳感器。

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<span =“apple-style-span” style=“font-size: large;”>When using a pseudodifferential input, the negative input should not vary much with time. The negative input should be a reference only. Another way of describing pseudodifferential inputs is that the inputs are differential only in the sense that ground loops and DC common-mode voltage are broken. The reference signal, (negative input), is not intended to carry signals of interest but only to provide a DC reference point for the signal (positive input). Pseudodifferential inputs minimize the effects of ground potential differences (ground loops) between the signal source and the device, which leads to more accurate measurements.

<span =“apple-style-span” style=“font-size: large;”>當使用偽輸入,負輸入應隨時間變化不大。負輸入應該是僅供參考。另一種方式描述偽投入,是投入差別只在意義上,接地迴路和直流共模電壓的損壞。該參考信號(負輸入),是不是要進行信號的興趣,但只提供一個直流參考點的信號(正輸入)。偽投入減少的影響,地面電位差(接地迴路)與信號源和設備,從而導致更多的精確測量。

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<span =“apple-style-span” style=“font-size: large;”>Choosing Channel Configurations

<span =“apple-style-span” style=“font-size: large;”>Configure the channels based on how the signal source or device under test (DUT) is referenced. If the signal source is floating, use the pseudodifferential channel configuration. A floating signal source does not connect to the building ground system. Instead, the signal source has an isolated ground-reference point. Some examples of floating signal sources are outputs of transformers without grounded center taps, battery-powered devices, non-grounded accelerometers, and most instrumentation microphones. An instrument or device that has an isolated output is considered a floating signal source. It is important to provide a ground reference for a floating signal. If no ground-reference point is provided—for example, selecting differential mode with a floating microphone—the microphone outputs can drift outside of the device common-mode range.

<span =“apple-style-span” style=“font-size: large;”>配置渠道的基礎上如何信號源或設備被測設備(DUT)的引用。如果信號源是浮動的,使用偽通道配置。浮動信號源沒有連接到建築物的地面系統。相反,信號源有一個孤立的地面參考點。一些例子浮動信號源沒有輸出變壓器中心抽頭接地,電池供電設備,非接地加速度計,麥克風和最儀器。的儀器或設備,有一個隔離輸出被認為是浮動信號源。重要的是要提供一個參考地為浮動信號。如果沒有地面參考點提供的,例如,選擇不同模式,一個浮動麥克風麥克風輸出可漂裝置以外的共模範圍。

<span =“apple-style-span” style=“font-size: large;”>If the signal source is ground-referenced, use either the differential or pseudodifferential channel configurations. A ground-referenced signal source connects in some way to the building ground. Therefore, it is already connected to a ground-reference point with respect to the measurement device, assuming the PXI or CompactPCI chassis and controller are plugged into the same power system. Non-isolated outputs of instruments and devices that plug into the building power system fall into this category.

<span =“apple-style-span” style=“font-size: large;”>如果信號源是地面參考,使用或偽差分通道配置。阿地為基準的信號源以某種方式連接到地面的建築物。因此,它已經連接到接地參考點就到測量設備,假定PXI或CompactPCI的機箱和控制器插入到相同的電源系統。非隔離輸出的儀器和設備插入到電力系統的建設屬於這一類。

<span =“apple-style-span” style=“font-size: large;”>Provide only one ground-reference point for each channel by properly selecting differential or pseudodifferential configuration. If you provide two ground-reference points—for example, if you select pseudodifferential mode with a grounded accelerometer—the difference in ground potential results in currents in the ground system that can cause measurement errors. The 50 ohm resistor on the signal ground is usually sufficient to reduce this current to negligible levels, but results can vary depending on the system setup. The table below summarizes the configuration for various types of signal sources.

<span =“apple-style-span” style=“font-size: large;”>只提供一個地面基準點的正確選擇每個通道差分或偽配置。如果您提供了兩個地面參考點,例如,如果您選擇偽模式,接地的加速度計,在地電位的差異導致電流接地系統中可能導致的測量誤差。 50歐姆的電阻的信號地面通常足以減少這種電流可以忽略的水平,但結果可能取決於系統設置。下表匯總了配置各類信號源。

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Channel Configuration

Pseudodifferential

Differential

Source Reference

Floating

Recommended

Requires additional connection to ground to prevent drift beyond common-mode range

Ground-Referenced

Can be used, but differential  provides more common-mode rejection

Recommended

最近使用PCI-6251跟cRIO-9333中,發現有Differential, Pseudodifferential兩種不同的輸入方式。


覺得很納悶,就上網找一些資料,參考一下。