標稱-48V的電源廣泛地用于無線基站和其他網(wǎng)絡(luò)中心站的通訊設(shè)備，能在-48V到-60V范圍內(nèi)變化。測量其電流典型地需要±15V雙電源供電的器件。不使用負電源可以減少系統(tǒng)的復(fù)雜程度和費用。本設(shè)計方案使用Analog Devices公司的AD629差分放大器和AD8603運算放大器，測量-48V到-60V的電流，且僅需要單一正電源供電（參考文獻1和2）。
圖1顯示了AD629和AD8603如何在-48V共模電壓面前測量電流。下面的公式表明AD629差分放大器能調(diào)節(jié)超過電源范圍的電壓：VCOM_MAX=20×(VS–1.2)–19×VREF, 和VCOM_MIN=20×(–VS+1.2)–19×VREF。共模輸入范圍為–71到+121V，帶一個5V參考電壓。電流I流過分流電阻Rs，由差動放大器形成微分電壓。AD629固定為1倍增益，那么輸出電壓為I×RS+VREF。AD8603執(zhí)行減法器的功能，所以可以抵消共模電壓VREF，將增益應(yīng)用到有用信號I×RS。以20因數(shù)放大信號到ADC的2.5V全量程范圍。
本設(shè)計方案使用AD8603，因為其具有低輸入偏置電流和低偏移量漂移。此外，軌對軌輸出允許其和ADC分享同一個電源。這個階段，減法器抵消了電壓參考的5V共模信號。形成減法器的四個電阻必須具有與獲得最大共模抑制比相匹配的比例。如果不能得到適合的匹配電阻，可以使用AD623單電源儀表放大器替代AD8603，確保高共模抑制比。
兩個放大器的偏移量、輸入偏置電流和共模抑制比誤差，導(dǎo)致AD8603輸出的最大誤差為163mV。這個計算假設(shè)電阻為0.01%的匹配率。該電路經(jīng)歷過并聯(lián)50、100和200mΩ的Rs來檢證。
英文原文：
Single-supply circuit measures –48V high-side current
This circuit measures ?48V high-side current using a single positive supply.
Wenshuai Liao, Analog Devices, Beijing, China; Stephen Lee, Analog Devices, Wilmington, MA; and Yanhui Zhao, Beihang University, Beijing, China; Edited by Charles H Small and Fran Granville -- EDN, 2/21/2008
The nominal –48V rail, which finds wide use in wireless base stations and other telecommunications equipment in network central offices, can vary from –48 to –60V. Measuring its current draw typically requires components that operate on ±15V dual supplies. Eliminating the negative supply would reduce system complexity and cost. This Design Idea uses an AD629 difference amplifier and an AD8603 operational amplifier, both from Analog Devices, to measure current at –48 to –60V and operates from a single positive-power supply (reference 1 and reference 2).
Figure 1 shows how the AD629 and AD8603 measure current in the presence of a –48V common-mode voltage. The following equations demonstrate how the AD629 difference amplifier can condition voltages beyond its supply ranges: VCOM_MAX=20×(VS–1.2)–19×VREF, and VCOM_MIN=20×(–VS+1.2)–19×VREF. With a 5V reference, the common-mode input range is –71 to +121V. The current, I, flows through the shunt resistor, RS, causing a differential voltage, which the difference amplifier senses. The AD629 has a fixed gain of one, so the output voltage is I×RS+VREF. The AD8603 functions as a subtractor so that it can reject the common-mode voltage, VREF, and apply gain to the signal of interest, I×RS. A factor of 20 amplifies the signal to span the 2.5V full-scale range of the ADC.
This Design Idea uses the AD8603 because it has low input-bias current and low offset drift. In addition, the rail-to-rail output allows it to share the same supply as the ADC. In this stage, the subtractor rejects the 5V common-mode signal from the voltage reference. The four resistors that form the subtractor must have matched ratios to obtain maximum common-mode rejection. If you cannot obtain tightly matched resistors, you can use an AD623 single-supply instrumentation amplifier in place of the AD8603, ensuring high common-mode rejection.
Offset, input-bias-current, and common-mode-rejection errors from both amplifiers result in a 163-mV maximum error at the output of the AD8603. This calculation assumes resistors with a 0.01% ratio match. The circuit was verified on the bench using 50-, 100-, and 200-mΩ shunts for RS.
References
“High Common-Mode Voltage, Difference Amplifier AD629,” Analog Devices, 1999 to 2007.
“Precision Micropower, Low Noise CMOS Rail-to-Rail Input/Output Operational Amplifiers AD8603/AD8607/AD8609,” Analog Devices, 2005.