20位数字模拟转换器的使用

　　简介

　　高精度，仪器仪表的重大进展等级A转换器最近发生。十年前12位的数-模转换器（DAC）被认为是高级设备。今天，16位DAC可在系统设计中越来越普遍。这些都是真的不到1lsb线性误差和精密设备1ppm的/°C漂移。1然而，在DAC的应用这需要更高的性能。自动测试设备、仪器、校准装置、激光机，医疗电子和其他应用经常要求DAC精度超过16位。18位DAC有在电路装配形式，虽然他们是价格昂贵，需要经常校准。20、即使23 +（0.1ppm！）位DAC由手动交换Kelvin Varley分频器。这些设备，虽然惊人的准确，大，慢，非常昂贵。他们的使用通常只限于标准实验室。2有用的发展将是一个现实，20位（1ppm）DAC很容易构造，不需要频繁的校准。

　　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.