10kΩ是什么顏色?

　　故事來源于The Cube：編碼把握關(guān)鍵到用合適的電阻建立電路板。

　　80年代，我在生產(chǎn)STDBus格式的工業(yè)計(jì)算機(jī)公司工作。（它很快成為單板計(jì)算機(jī)格式。）我的第一個(gè)主要設(shè)計(jì)是移植ADC/DAC多I/O卡的ADC由12位到14位?？ㄔO(shè)計(jì)很靈活：它能處理16個(gè)單端或8個(gè)差分輸入，每個(gè)輸入范圍為0到5V、0到10V、±5V、或±10V，帶熱電偶板上冷端。ADC輸入和8輸出12位DAC分別可編程。轉(zhuǎn)換和升級到100KHz范圍，所有由8051帶64kb EEPROM和32kb SRAM控制。迄今，一切正常。

　　我發(fā)現(xiàn)多源ADC和做一些前端工作——輸入運(yùn)放LM324和誤差預(yù)算是寬松的。幾乎同樣價(jià)格，較低偏置和較低溫度漂移器件有許多選擇。編程的設(shè)計(jì)不是直覺，使用者手冊有一些提示。仔細(xì)檢查管腳和原型構(gòu)造正常，但當(dāng)燒進(jìn)板子，板子提供11位可靠數(shù)據(jù)，偶爾12位。我最終將問題定位到微處理器在模數(shù)轉(zhuǎn)換期間的擾動(dòng)。我們換了CMOS 8051，不使用數(shù)字部分，讓“轉(zhuǎn)換結(jié)束”標(biāo)志喚醒微處理器。現(xiàn)在，整個(gè)輸入范圍內(nèi)有13到14位可靠數(shù)據(jù)，可以真實(shí)地鼓吹商業(yè)雜志中提到的更好的分辨力。訂單隨之而置，銷售高

興，生活美好。

　　一切正常，直到我們不能適當(dāng)?shù)臏y試和校準(zhǔn)第一批的25個(gè)板子?，F(xiàn)在，顧客和銷售開始心急如焚。顯微鏡、儀表、替換器件、研究芯片，檢查印制板——每個(gè)設(shè)計(jì)工程師熟知的調(diào)試工作——全都沒用。我的手工板工作正常，但是所有的產(chǎn)品都測試失敗。

　　我們使用了4051類型通道可選的數(shù)據(jù)鎖存，有規(guī)格化增益設(shè)置電阻約為10kΩ。值足夠大到淹沒更換電阻的改變，低到削弱電阻噪聲，高到保持偏置電流和自發(fā)熱較低的狀態(tài)。加上，它使算術(shù)容易。（在那些天，我們?nèi)允褂弥辈迤骷＃?/P>

　　盯著好和壞的板子，最終我恍然大悟。每個(gè)優(yōu)秀的設(shè)計(jì)工程師都知道，10kΩ電阻編碼是按順序到公差帶為棕－黑－橙。壞板子將10kΩ和300Ω電阻弄混了！我盯著問題長時(shí)間，以為是一些很大的錯(cuò)誤，但錯(cuò)誤卻如此明顯。回溯破壞足跡解開了整體故事。人們創(chuàng)建整體，不知道電阻代碼來源于Morse碼；無論裝了什么，他們正確選擇箱柜。倉庫使用工具箱引入檢查。我的計(jì)算機(jī)從發(fā)行人手中買，保留對多于2%的電阻進(jìn)行測試。我檢查發(fā)現(xiàn)60/40%弄混了10kΩ和300Ω電阻。我們感謝沒有箱柜致于相同的混亂。我們和買主、工程人員和產(chǎn)品團(tuán)隊(duì)，心對心的交談。我認(rèn)為大家都知道，沒人了解電阻顏色代碼，但每個(gè)人都知道怎么識別正確的器件。

　　英文原文：

　　What color is 10 kΩ?

　　Tales From The Cube: A code held the key to building a board with the right resistors.

　　By John Linstrom, SK Electronics and Sunspot -- EDN, 11/8/2007

　　In the ’80s, I worked for a company that made industrial computers in an STDBus format. (It soon went to a single-board-computer format.) My first major project was to migrate the ADC on its ADC/DAC multi-I/O card from 12 to 14 bits. The card design was flexible: It could handle 16 single-ended or eight differential inputs, each over a range of 0 to 5V, 0 to 10V, ±5V, or ±10V, with an onboard cold junction for thermocouples. The ADC inputs and eight-output, 12-bit DAC were individually strap-programmable. Conversions and updates were in the 100-kHz range, all under the control of an 8051 with a 64-kbyte EEPROM and 32-kbyte SRAM. So far, so good.

　　I found a multisourced ADC and did some front-end work—the input amps were LM324s, and the error budget was wide. There were many choices for lower-offset and lower-temperature-drift parts at nearly the same price. The layout of the programming straps was not intuitive, and the user manual got a few tweaks. We went through an artwork spin and prototype build just fine, but when we fired up the board, it gave us 11 bits of solid data, maybe 12 on a good day. I finally narrowed down the problem to microprocessor churning during analog-to-digital-conversion cycles. We switched to a CMOS 8051, put the digi

tal side to sleep, and let the “end-of-conversion” flag wake up the microprocessor. Now, the team had 13 to 14 bits of solid data over the whole input range and could honestly brag about greater resolution in the trade magazines. Orders followed, salesmen were happy, and life was good.

　　It was good, that was, until we couldn’t properly test and calibrate the first batch of 25 boards. Now, the eager customers and smiling salesmen started turning up the heat. The team scoped, metered, substituted parts, studied the artwork, and checked the golden board—following the drill every design engineer knows by heart—all for naught. My handmade board worked great, but all the production copies tested in the weeds.

　　We used 4051-style data latches for the channel selects, and I had normalized-gain-set resistors with values of approximately 10 kΩ. The value was high enough to swamp any changes in switch resistance, low enough to keep down resistor noise, and high enough to keep bias currents and self-heating low. Plus, it made the math easy. (In those days, we still used through-hole parts.)

　　Staring at the good and bad boards finally gave me the “smack upside the head.” The 10-kΩ-resistor code, as every good design engineer knows, is brown-black-orange followed—you hope—by a tolerance band. The bad boards were stuffed with a mix of 10-kΩ and 300Ω resistors! I had stared at the problem so long, expecting something immense, that I had blown right by the obvious. Backtracking the trail of destruction unraveled the whole tale. The people who built the units didn’t know resistor codes from Morse code; they stuffed in whatever was in the correct pick bins. The stockroom used kits from bins that incoming inspection filled. My computer bought from reputable distributors but reserved incoming tests for items more costly than 2-cent resistors. I checked the stocking bins and found a 60/40% mix of 10-kΩ and 300Ω resistors. We were thankful that no other bins fell prey to the same confusion. We had a heart-to-heart talk with the buyers, engineering staff, and production team. I don’t think anyone learned the
resistor color code that didn’t already know it, but everyone learned how to identify correct parts.