電(dian)(dian)流(liu)(liu)表(biao)(biao)是用于測量(liang)電(dian)(dian)流(liu)(liu)設備，讀(du)數為安培（amperes）。存在兩種電(dian)(dian)流(liu)(liu)表(biao)(biao)結構(gou)：分流(liu)(liu)（shunt）型電(dian)(dian)流(liu)(liu)表(biao)(biao)和反饋（feedback）型電(dian)(dian)流(liu)(liu)表(biao)(biao)。

　　1、分流型與反饋型電流表

　　分流(liu)型(xing)(xing)電(dian)流(liu)表是常用的(de)形式，應(ying)(ying)用在很(hen)多場合(he)。反饋型(xing)(xing)安培表更加(jia)適合(he)測(ce)(ce)量小(xiao)(xiao)電(dian)流(liu)，隨著如今測(ce)(ce)量電(dian)流(liu)越(yue)(yue)來越(yue)(yue)小(xiao)(xiao)，反饋型(xing)(xing)電(dian)流(liu)表應(ying)(ying)用逐步增加(jia)。然(ran)而(er)選擇正(zheng)確(que)形式的(de)電(dian)流(liu)表不僅取決于被測(ce)(ce)電(dian)流(liu)幅值的(de)大小(xiao)(xiao)，也與被測(ce)(ce)設備(bei)（DUT）的(de)特征（通常指阻(zu)抗）有關系。

　　2、分流電流表：DMMs

　　常見的分流(liu)(liu)型電(dian)(dian)流(liu)(liu)表(biao)在幾(ji)乎所有數字萬(wan)用表(biao)（DMMs）中得到應用。被測(ce)量(liang)的電(dian)(dian)流(liu)(liu)在萬(wan)用表(biao)輸入(ru)端電(dian)(dian)阻上(shang)形成電(dian)(dian)壓正比于測(ce)量(liang)電(dian)(dian)流(liu)(liu)。

▲ 圖1  分流型電流表

　　分流型電(dian)流表(biao)的主要缺點在于(yu)電(dian)流表(biao)的輸(shu)入阻(zu)抗。當輸(shu)入電(dian)流減(jian)小的時這個確定(ding)越發明顯，因為需要更大的輸(shu)入電(dian)阻(zu)才能夠產生更高的被測電(dian)壓。分流型電(dian)流表(biao)一般(ban)在以(yi)下兩(liang)種情況(kuang)工(gong)作良好：

　　電(dian)流表的(de)輸(shu)入阻(zu)抗(kang)比(bi)起待(dai)測設備(bei)的(de)輸(shu)出阻(zu)抗(kang)要小得多；

　　被(bei)測電流不要太小(xiao)，不要比微安（）小(xiao)的(de)太多；

　　3、電壓負擔

　　電(dian)(dian)(dian)(dian)流表的端口電(dian)(dian)(dian)(dian)壓(ya)被稱為(wei)電(dian)(dian)(dian)(dian)壓(ya)負擔（Voltage Borden）。隨著電(dian)(dian)(dian)(dian)流表串入待測電(dian)(dian)(dian)(dian)流回路，電(dian)(dian)(dian)(dian)壓(ya)負擔則會引(yin)起被測電(dian)(dian)(dian)(dian)流減小，因此電(dian)(dian)(dian)(dian)流表就無法(fa)準確反映(ying)被測電(dian)(dian)(dian)(dian)流。

　　一(yi)個(ge)理(li)想(xiang)的(de)電(dian)(dian)流(liu)(liu)表(biao)(biao)應該對(dui)電(dian)(dian)流(liu)(liu)回路中的(de)電(dian)(dian)流(liu)(liu)不產生任何影響，所以它需要(yao)有零(ling)輸入阻(zu)抗和(he)零(ling)電(dian)(dian)壓(ya)負(fu)(fu)(fu)擔(dan)。但(dan)是一(yi)個(ge)實(shi)際的(de)電(dian)(dian)流(liu)(liu)表(biao)(biao)則(ze)會產生一(yi)定大小的(de)電(dian)(dian)壓(ya)負(fu)(fu)(fu)擔(dan)。一(yi)般來說，由電(dian)(dian)流(liu)(liu)表(biao)(biao)的(de)電(dian)(dian)壓(ya)負(fu)(fu)(fu)擔(dan)所產生的(de)測量(liang)誤差等于電(dian)(dian)壓(ya)負(fu)(fu)(fu)擔(dan)除以被測設備輸出電(dian)(dian)阻(zu)。典型(xing)的(de)電(dian)(dian)流(liu)(liu)表(biao)(biao)所產生的(de)電(dian)(dian)壓(ya)負(fu)(fu)(fu)擔(dan)量(liang)級在數百毫伏左右(you)。

　　4、反饋型電流表

　　相比(bi)于分流(liu)型，反饋型電(dian)流(liu)表(biao)更(geng)接近于理(li)想電(dian)流(liu)表(biao)，在測(ce)量(liang)微安級別(bie)以下的(de)電(dian)流(liu)時應該選擇這種電(dian)流(liu)表(biao)，或者在要(yao)求輸(shu)入(ru)阻抗有特別(bie)苛刻要(yao)求的(de)情況下。

　　反(fan)(fan)饋型(xing)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)是(shi)通(tong)(tong)過一個(ge)增(zeng)益(yi)很高的(de)(de)(de)運算放大器的(de)(de)(de)反(fan)(fan)饋回路來將輸(shu)入(ru)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)轉(zhuan)換成電(dian)(dian)(dian)(dian)(dian)(dian)壓，電(dian)(dian)(dian)(dian)(dian)(dian)壓與被(bei)測電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)成正比；這個(ge)電(dian)(dian)(dian)(dian)(dian)(dian)壓不再出現在電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)的(de)(de)(de)輸(shu)入(ru)端口，而是(shi)通(tong)(tong)過放大器輸(shu)出到(dao)(dao)(dao)后級測量電(dian)(dian)(dian)(dian)(dian)(dian)路。電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)的(de)(de)(de)輸(shu)入(ru)電(dian)(dian)(dian)(dian)(dian)(dian)壓（也(ye)就是(shi)前面提到(dao)(dao)(dao)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓負(fu)擔）等于(yu)輸(shu)出電(dian)(dian)(dian)(dian)(dian)(dian)壓除以(yi)運算放大器的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓增(zeng)益(yi)（通(tong)(tong)常為(wei)100,000），所以(yi)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)輸(shu)入(ru)端的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓負(fu)擔會(hui)減小到(dao)(dao)(dao)微(wei)伏級別。由于(yu)電(dian)(dian)(dian)(dian)(dian)(dian)壓負(fu)擔小，所以(yi)反(fan)(fan)饋型(xing)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)在測量小電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)，或者被(bei)測設備輸(shu)出阻(zu)抗小的(de)(de)(de)時候(hou)，所產生的(de)(de)(de)誤(wu)差更(geng)小。Keithley電(dian)(dian)(dian)(dian)(dian)(dian)子電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)和(he)p-A電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)都采用反(fan)(fan)饋型(xing)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)表(biao)技(ji)術。

▲ 圖2  反(fan)饋(kui)型電流表

　　下圖對(dui)測(ce)(ce)量三極(ji)管(guan)發射(she)極(ji)電(dian)流時(shi)，電(dian)壓(ya)負擔所(suo)帶(dai)來的影響(xiang)。雖然使用(yong)(yong)數字萬用(yong)(yong)表可以(yi)很好的測(ce)(ce)量到(dao)三極(ji)管(guan)的發射(she)極(ji)電(dian)流，但數字萬用(yong)(yong)表的電(dian)壓(ya)負擔會顯著(zhu)減(jian)小(xiao)被測(ce)(ce)隊形的電(dian)壓(ya)，使得測(ce)(ce)量結果(guo)偏小(xiao)。如果(guo)使用(yong)(yong)p-A表或者(zhe)電(dian)子電(dian)流表，測(ce)(ce)量誤(wu)差可以(yi)忽略不(bu)計。

　　▲圖3測量三極(ji)管發射極(ji)電流

　　5、產生電流誤差原因

　　有(you)若干(gan)原因會(hui)引(yin)起測(ce)(ce)量電(dian)(dian)流(liu)(liu)的(de)精度(du)。所有(you)的(de)電(dian)(dian)流(liu)(liu)表都會(hui)產(chan)生(sheng)一(yi)個小(xiao)的(de)電(dian)(dian)流(liu)(liu)流(liu)(liu)過輸入(ru)端，即(ji)使輸入(ru)端在開路狀態，這種電(dian)(dian)流(liu)(liu)被稱為偏置電(dian)(dian)流(liu)(liu)，可以通過設(she)備(bei)電(dian)(dian)流(liu)(liu)抑(yi)制來進行調零(ling)。外部泄露電(dian)(dian)流(liu)(liu)是附加的(de)測(ce)(ce)量誤差，通過良好(hao)的(de)保護和屏蔽連接對于(yu)減(jian)(jian)少泄露電(dian)(dian)流(liu)(liu)非常重(zhong)要。被測(ce)(ce)設(she)備(bei)的(de)輸出阻抗(kang)影(ying)響電(dian)(dian)流(liu)(liu)表的(de)噪聲性能。除此之外，還有(you)一(yi)些(xie)其它產(chan)生(sheng)電(dian)(dian)流(liu)(liu)也(ye)會(hui)疊加在被測(ce)(ce)電(dian)(dian)流(liu)(liu)上引(yin)起誤差。下面討(tao)論(lun)一(yi)些(xie)產(chan)生(sheng)電(dian)(dian)流(liu)(liu)種類以及如何(he)減(jian)(jian)少它們對測(ce)(ce)量的(de)影(ying)響。

　　（1）摩擦電流

　　摩(mo)(mo)擦電(dian)是由于導(dao)線(xian)與(yu)絕緣(yuan)層(ceng)(ceng)之間的電(dian)荷(he)不平衡引起的，如下圖所示。選擇合適(shi)的導(dao)線(xian)，比如Keithely低(di)噪聲電(dian)纜可以(yi)減少這種摩(mo)(mo)擦電(dian)的影響，它內部的聚乙(yi)烯絕緣(yuan)層(ceng)(ceng)外(wai)包(bao)括有一層(ceng)(ceng)石(shi)墨，起到與(yu)外(wai)部屏蔽層(ceng)(ceng)之間的潤滑作用，在(zai)導(dao)線(xian)外(wai)產生圓桶形狀的等電(dian)位保護層(ceng)(ceng)，均衡電(dian)荷(he)分布減少電(dian)荷(he)產生。

　　▲圖4摩(mo)擦起電對電流測量影響

　　（2）壓電電流

　　當一些特(te)殊(shu)晶體結構物體受到機械(xie)壓力時(shi)會產(chan)生(sheng)壓電(dian)(dian)電(dian)(dian)流(liu)，通常發生(sheng)在絕緣接(jie)口(kou)端子(zi)和內部連接(jie)硬件上。一些塑(su)料(liao)中，內部可能存(cun)儲有電(dian)(dian)荷，它們可能產(chan)生(sheng)類似于壓電(dian)(dian)效應。下圖顯示了具(ju)有壓電(dian)(dian)效應的端口(kou)所產(chan)生(sheng)的壓電(dian)(dian)電(dian)(dian)流(liu)。可以通過以下兩種方法來(lai)減少壓電(dian)(dian)電(dian)(dian)流(liu)：

　　消除(chu)絕緣材(cai)料上所受到(dao)的機械(xie)壓(ya)力；

　　使用壓電(dian)效應小，或者(zhe)存儲電(dian)荷(he)少的絕緣材(cai)料；

　　▲圖5具有壓電效(xiao)應(ying)的接口端子

　　（3）污染和潮濕

　　污染物再加上潮濕會引起電化學效應，從而產生誤差電流，這是因為具有電離化學物質的污染物會在電路板兩個導體之間產生弱的“電池”效應。例如，在常用的環氧樹脂印刷電路板上，如果沒有把腐蝕溶液、助焊劑、油漬、鹽（通常來自于指紋）或者其他污染物進行徹底清理，就會在兩個導體之間產生若干n-A電流，如下圖所示。避免這種由污染物和潮濕所引起的誤差，選擇不吸水的絕緣材料，或者保持空氣濕度在一個合適的水平。對所有的絕緣材料保持它們的清潔，避免被污染物沾染。

　　▲圖6污染物在電路導體之間產生的誤差(cha)電流

　　6、測量高阻抗

　　在(zai)測(ce)(ce)(ce)量高(gao)(gao)阻(zu)抗（通常大于(yu)(yu)1GΩ）時，使(shi)用(yong)一(yi)個恒(heng)定(ding)(ding)的(de)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)加(jia)載待測(ce)(ce)(ce)電(dian)(dian)(dian)(dian)(dian)阻(zu)上，然(ran)后(hou)通過串聯電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)表(biao)(biao)(biao)檢測(ce)(ce)(ce)所產(chan)生的(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)。使(shi)用(yong)歐(ou)姆定(ding)(ding)律（R=V/I）可以(yi)計算出電(dian)(dian)(dian)(dian)(dian)阻(zu)值。這種施(shi)加(jia)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)去測(ce)(ce)(ce)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)比施(shi)加(jia)恒(heng)流(liu)(liu)測(ce)(ce)(ce)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)的(de)方(fang)法更適(shi)合于(yu)(yu)高(gao)(gao)阻(zu)測(ce)(ce)(ce)量，這是由于(yu)(yu)一(yi)些高(gao)(gao)阻(zu)電(dian)(dian)(dian)(dian)(dian)阻(zu)的(de)特(te)性有可能(neng)會受到高(gao)(gao)壓(ya)(ya)的(de)影響。當然(ran)一(yi)個受控(kong)且可以(yi)產(chan)生合適(shi)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)范圍(wei)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)源對于(yu)(yu)測(ce)(ce)(ce)量非常重要(yao)。這種方(fang)法通常需要(yao)使(shi)用(yong)到能(neng)夠(gou)測(ce)(ce)(ce)量小(xiao)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)的(de)電(dian)(dian)(dian)(dian)(dian)子電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)表(biao)(biao)(biao)或(huo)者p-A表(biao)(biao)(biao)。前面關(guan)于(yu)(yu)小(xiao)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)測(ce)(ce)(ce)量及其誤差分析的(de)內容(rong)都適(shi)合在(zai)高(gao)(gao)阻(zu)抗測(ce)(ce)(ce)量中。

　　在測(ce)(ce)量高阻(zu)時，泄(xie)露(lu)電流是常(chang)見到的(de)誤差來源(yuan)，它們產生(sheng)于待測(ce)(ce)電路與附(fu)近的(de)電壓源(yuan)之(zhi)間的(de)高阻(zu)抗通(tong)道(dao)（泄(xie)露(lu)電阻(zu)）。通(tong)過使用合適(shi)的(de)保護特性(xing)、清潔、高質量絕緣材料以及(ji)減(jian)少(shao)濕度等措(cuo)施來減(jian)少(shao)泄(xie)漏電阻(zu)的(de)影響(xiang)。

　　下(xia)圖給(gei)出了常見到的(de)(de)絕緣材(cai)料的(de)(de)電阻值。如果吸收了水(shui)分(fen)則會使(shi)得部分(fen)絕緣材(cai)料的(de)(de)電阻改變若干的(de)(de)數量級。

　　▲圖(tu)7不同絕緣材料的電阻

　　下面表格給出(chu)了水分吸收以及其(qi)他效(xiao)應的定量描述(shu)。

　　▲圖8不同絕緣材料(liao)的電(dian)阻值

　　▲圖9材料的吸水、壓電(dian)、摩擦起電(dian)特性

　　7、交替極性方法

　　當測(ce)(ce)量物質(zhi)具有很高的阻(zu)抗時(shi)，背景電流(liu)可能引起顯著(zhu)的測(ce)(ce)量誤(wu)差，它(ta)們可能是由絕緣材料(liao)中儲(chu)存電荷、靜電或者摩擦(ca)起電、壓(ya)電效應等引起的。

　　使(shi)用(yong)交(jiao)替(ti)極(ji)性測(ce)量(liang)方(fang)法可以(yi)(yi)將樣品中(zhong)背景電(dian)(dian)(dian)(dian)流(liu)對測(ce)量(liang)結果的(de)(de)影(ying)響消除。這種方(fang)法也很簡單，就(jiu)是對被測(ce)電(dian)(dian)(dian)(dian)阻(zu)正向施加(jia)電(dian)(dian)(dian)(dian)壓，在一段延時(shi)后測(ce)量(liang)流(liu)過電(dian)(dian)(dian)(dian)流(liu)；然(ran)后再(zai)改(gai)變(bian)電(dian)(dian)(dian)(dian)壓的(de)(de)極(ji)性，延時(shi)一段時(shi)間后再(zai)測(ce)量(liang)電(dian)(dian)(dian)(dian)流(liu)。使(shi)用(yong)電(dian)(dian)(dian)(dian)壓的(de)(de)變(bian)化(hua)值(zhi)除以(yi)(yi)電(dian)(dian)(dian)(dian)流(liu)的(de)(de)變(bian)化(hua)值(zhi)來計算被測(ce)電(dian)(dian)(dian)(dian)阻(zu)的(de)(de)大(da)小。這種方(fang)法可以(yi)(yi)重(zhong)復若干次，取測(ce)量(liang)值(zhi)的(de)(de)加(jia)權(quan)平(ping)均值(zhi)來反映被測(ce)電(dian)(dian)(dian)(dian)阻(zu)值(zhi)。