簡介

　　高精度，儀器儀表的重大進(jìn)展等級A轉(zhuǎn)換器最近發(fā)生。十年前12位的數(shù)-模轉(zhuǎn)換器（DAC）被認(rèn)為是高級設(shè)備。今天，16位DAC可在系統(tǒng)設(shè)計中越來越普遍。這些都是真的不到1lsb線性誤差和精密設(shè)備1ppm的/°C漂移。1然而，在DAC的應(yīng)用這需要更高的性能。自動測試設(shè)備、儀器、校準(zhǔn)裝置、激光機(jī)，醫(yī)療電子和其他應(yīng)用經(jīng)常要求DAC精度超過16位。18位DAC有在電路裝配形式，雖然他們是價格昂貴，需要經(jīng)常校準(zhǔn)。20、即使23 +（0.1ppm！）位DAC由手動交換Kelvin Varley分頻器。這些設(shè)備，雖然驚人的準(zhǔn)確，大，慢，非常昂貴。他們的使用通常只限于標(biāo)準(zhǔn)實驗室。2有用的發(fā)展將是一個現(xiàn)實，20位（1ppm）DAC很容易構(gòu)造，不需要頻繁的校準(zhǔn)。

　　20-Bit DAC Architecture

　　Figure 1 diagrams the architecture of a 20-bit （1ppm） DAC. This scheme is based on the availability of a true 1ppm analog-to-digital converter with scale and zero drifts below 0.02ppm/°C. This device， the LTC? 2400， is used as a feedback element in a digitally corrected loop to realize a 20-bit DAC.3

　　In practice， the “slave” 20-bit DAC’s output is monitored by the “master” LTC2400 A-to-D， which feeds digital information to a code comparator. The code comparator differences the user input word with the LTC2400 output， presenting a corrected code to the slave DAC. In this fashion， the slave DAC’s drifts and nonlinearity are continuously corrected by the loop to an accuracy determined by the A-to-D converter and VREF4. The sole DAC requirement is that it be monotonic. No other components in the loop need to be stable.