摘要：數(shù)字電位器與機(jī)械式電位器相比具有許多優(yōu)勢(shì)，例如，具有更低的系統(tǒng)成本、能夠改善系統(tǒng)的可靠性以及提高系統(tǒng)的靈活性等。本文介紹了使用數(shù)字電位器的優(yōu)、缺點(diǎn)，并對(duì)兩者進(jìn)行了性能對(duì)比。

機(jī)械式電位器(電位計(jì))是可變電阻，長(zhǎng)期用來(lái)調(diào)節(jié)系統(tǒng)的失調(diào)和增益、設(shè)置LCD的對(duì)比度電壓、調(diào)節(jié)電源電壓 ― 這些只是眾多應(yīng)用中的少數(shù)幾種。目前，利用螺絲刀進(jìn)行模擬調(diào)節(jié)的方式正逐步被淘汰，這種方式的成本較高，而且存在人為誤差。針對(duì)當(dāng)前市場(chǎng)，用數(shù)字電位器替代機(jī)械式電位器將會(huì)為產(chǎn)品帶來(lái)極大的競(jìng)爭(zhēng)力。數(shù)字電位器能夠提高產(chǎn)品的可靠性，另外，由于消除了高成本、存在一定問(wèn)題的手工調(diào)節(jié)，簡(jiǎn)化了生產(chǎn)流程?？梢院苋菀椎亟档彤a(chǎn)品成本，提高裝配速度。通常情況下，用數(shù)字電位器替代機(jī)械式電位器，可以得到完美的解決方案，但在考察系統(tǒng)是否適合這種升級(jí)時(shí)需要注意幾個(gè)方面的問(wèn)題。

分辨率

機(jī)械式電位器的分辨率從理論上講是無(wú)限的，但實(shí)際效果則取決于調(diào)節(jié)人員的熟練程度。具體的熟練程度因人而異，不同時(shí)間也會(huì)得到不同結(jié)果，有時(shí)實(shí)際得到的分辨率會(huì)很低。調(diào)節(jié)一個(gè)“1圈”的電位器時(shí)很容易產(chǎn)生抖動(dòng)，而“10圈”的電位器則會(huì)減少抖動(dòng)。

數(shù)字電位器無(wú)法提供無(wú)限的分辨率，但是，對(duì)以給定的應(yīng)用，如果選擇分辨率足夠高的數(shù)字電位器，則可表現(xiàn)出近似連續(xù)的性能。另外，數(shù)字電位器所提供的分辨率是經(jīng)過(guò)指標(biāo)測(cè)試的，非?？煽?，而且是有保證的。

限制

機(jī)械式電位器，滑動(dòng)端連接在一個(gè)電阻元件上，當(dāng)它沿著元件的長(zhǎng)度移動(dòng)時(shí)阻值發(fā)生變化。為了確保設(shè)備可靠工作，滑動(dòng)端在產(chǎn)品有效期內(nèi)必須保持與電阻元件的良好接觸。機(jī)械式電位器在密封的封裝內(nèi)可以為滑動(dòng)端提供良好的保護(hù)，但這無(wú)疑提高了產(chǎn)品的成本。另外，這種機(jī)械式連接從本質(zhì)上容易受振動(dòng)、沖擊、濕度和壓力等因素的影響。

數(shù)字電位器能夠提供高可靠性、高品質(zhì)的解決方案。當(dāng)然，這種方案在實(shí)際應(yīng)用中也會(huì)存在一定的限制。

數(shù)字電位器的抽頭和端點(diǎn)受限于電源電壓的擺幅，有些設(shè)計(jì)可能采用2.7V至5.5V電源供電，而有些設(shè)計(jì)則采用±15V供電。任何情況下，端點(diǎn)電壓必須保持在IC的工作電壓范圍以?xún)?nèi)。在改變?cè)O(shè)計(jì)時(shí)須注意確保端點(diǎn)電壓不超出電源電壓的范圍，必要時(shí)也可以使用簡(jiǎn)單的電阻分壓器。

數(shù)字電位器有易失和非易失兩種，對(duì)于易失電位器，一旦器件掉電，電位器的調(diào)節(jié)位置將丟失。這時(shí)，有可能需要使用外部EEPROM、閃存或其他類(lèi)型的非易失存儲(chǔ)器。非易失數(shù)字電位器芯片內(nèi)包含了這種存儲(chǔ)器，一般為EEPROM，在斷電時(shí)用于存儲(chǔ)電位器的設(shè)置。

此外，數(shù)字電位器需要在加電條件下進(jìn)行調(diào)節(jié)和工作，沒(méi)有加電的情況下不具備電阻功能。

比較機(jī)械式電位器與數(shù)字電位器的成本時(shí)，需綜合考慮實(shí)際使用成本。如果考慮到安裝成本、與物力調(diào)節(jié)有關(guān)的操作以及潛在的保修成本，數(shù)字電位器不愧為一種低成本的解決方案。在需要多個(gè)數(shù)字電位器的場(chǎng)合，成本還會(huì)進(jìn)一步降低。表1列出了數(shù)字電位器與機(jī)械式電位器之間的性能對(duì)比。

表1. 數(shù)字電位器與機(jī)械式電位器

Feature	Digital Pot	Mechanical Pot
Resolution	32 to 256 steps are available at this time.	Theoretically, this is infinite, though limited by the skill level of the pot adjuster.
Common-mode limitations	This is presently limited to the power-supply rails.	This is limited to the breakdown voltages of the wiper/dielectric.
Unpowered operation	This only functions when power is applied to the device.	These are functional without power, though utility may be limited.
Durability	The number of adjustment cycles is infinite, though nonvolatile devices may have EEPROM-write limitation.	The number of adjustment cycles is finite, and depend on the manufacturer/design.
Environmental concerns	Generally, these are very durable.	Depending on the design, some are very susceptible to vibration, shock, humidity, and pressure.
Setting retention	The setting is retained with the use of either external or included EEPROM/flash memory.	The setting is retained without power. More permanent retention can be obtained by the use of a glue dab, which is difficult to remove for subsequent adjustments.
Adjustment procedure	A 2-or 3-wire serial digital interface typically allows a MCU (or user) to adjust the pot. Also available are increment/decrement interfaces that do not require MCU intervention.	Typically a screwdriver is used to adjust the pot until the desired set point is read on a voltmeter or similar.
In-use pot adjustment	The MCU or user can adjust the pot as desired during operation. This can be helpful in automatically adjusting references, thresholds, etc.	If the set screw is not glued, a screwdriver may be used to adjust the pot manually.
Scaling	Linear and log are available.	Linear and log are available.
Temperature correction	Several digital pot designs include a temperature sensor and NV memory to provide a lookup-table-adjusted pot output over temperature.	Temperature-based adjustment is not possible.
Cost	The cost of the pot, plus the savings from eliminating a manual adjustment procedure and from lower rework/lower warranty repair cost, results in an overall cost savings.	Mechanical pots are inexpensive. However, the total overall cost will be higher due to the labor needed to make the adjustment, the possibility of adjustment error, and the cost of warranty repairs.

結(jié)論

幾乎在所有應(yīng)用中，用數(shù)字電位器替代機(jī)械式電位器都能夠改進(jìn)最終產(chǎn)品，有利于降低總體方案的成本，提高生產(chǎn)流程的可靠性和穩(wěn)定下。另外一個(gè)好處是，利用軟件可以實(shí)現(xiàn)系統(tǒng)的自動(dòng)調(diào)節(jié)、設(shè)置。是系統(tǒng)更加靈活、功能更廣泛。