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<H2 class=center>采用仪表放大器的0V压降分流器</H2>
<H3 class="center pt10px gray"><SPAN id=ctl00_Main_TechChannelArea>技术分类: <A
href="http://www.ednchina.com/TechClass/Analog/Default.aspx">模拟设计</A>
</SPAN>| 2009-02-01 <BR>Marian stofka,斯洛伐克,布拉迪斯拉发斯洛伐克技术大学</H3>
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<P> 无源分流器用于测量流经一个相对小阻值电阻的电流,一般对较大功率设备其满量程压降为60 mV,而对电子仪器为200
mV。与之类似,电流-电压转换器用于测量流经检测电阻的电流,它一般有更高的压降。但在某些情况下,输入端与地之间的压降必须尽可能低,0V为理想值(与被测电流无关)。如果你的应用需要这种特性,可以采用图1中的电流-电压转换器。此电路中,电阻R1用作一个经典的电流检测电阻,<A
title=仪表放大器 style="COLOR: blue; TEXT-DECORATION: underline"
href="http://article.ednchina.com/word/194012.aspx">仪表放大器</A>检测其上的被测电流,从而获得压降。该仪表放大器与R1不仅作为反相电流-电压转换器,而且也通过Point
B处的一个电阻网络建立了一个电压。这个电压幅度等于R1上的压降,极性与ΔVR1相反。最后的结果就是,输入端A上的电压值为理想的0V,无论流入输入端的电流幅度与极性如何。<BR><BR></P>
<P align=center><IMG height=316
alt=图1这只仪表放大器有两个用途它作为互阻为-5R的电流-电压转换器并在B点施加相反极性的一个压降因此无论输入电流是1或0输入端A电压都为零
hspace=0 src="采用仪表放大器的0V压降分流器 -- EDN电子设计技术.files/7-1.jpg" width=400
border=0></P>
<P> 设计采用了Analog
Devices公司的AD8223仪表放大器,因为它有缺省为5的电压增益,这个值以高精度接近于理想值。在缺省增益时的典型增益误差为0.03%,对B级IC的最差情况误差为0.1%(参考文献1)。当增益为5且R1与R2有相同值时,可以得出,对输入A的0V压降,R3的值是R2的两倍(图2)。图1中R1、R2和R3的都应是高精度、低温度系数型电阻。在R1和R2阻值为20Ω的实验电路中,存在着一个参照输入端0.8
?A的零漂电流,而在1 mA输入电流时,输入端A的压降变化为0.27
mV。对负输入电流,输入端A上会出现类似负电压变化。电路的传输常数(或互阻)为:(ΔVOUT)/(ΔIIN)=–5R。<BR><BR></P>
<P align=center><IMG height=179 alt=图2图1中输入端A为零压降时R3的值是R2的两倍 hspace=0
src="采用仪表放大器的0V压降分流器 -- EDN电子设计技术.files/7-2.jpg" width=300 border=0></P>
<P> 因此,如果输入电流为1 mA,则在输出端出现-100
mV电压。由于AD8223输出的拉入电流能力大约是供出电流能力的2.5倍,因此对正电流,输入范围也为2.5倍。将电源从±5V增加到±12V,就可以进一步增加正、负电流范围;也可以使用12V和-5V。如果你的设计需要更高的输入电流,就在仪表放大器输出和电阻R3之间放一个精密电压缓冲器,它要有相应的高输出电流能力。<BR><BR><FONT
color=#ff9900>参考文献<BR>1. “Single-Supply, Low-Cost Instrumentation Amplifier,
AD8223,” Analog Devices Inc, 2008. </FONT></P><!-- 2009-1-16 22:27:44-->
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