Precision Voltage References have been difficult to achieve using conventional process technology. This is evident by the high cost of such products. One measure of the need for a more precise reference is the existence of 12-16 bit ADC and DAC products that achieve integral linearity but are limited to only 10 bit absolute accuracy because of the voltage reference. In some applications better absolute accuracy is desirable. Consider the case of a 5 volt 12 bit converter operating over the industrial temperature range. If 100% of the converter error is allocated to the voltage reference then less than 0.5 LSB error over the 125°C range would be allowed. This would correspond to 122ppm or 0.012% or 0.61mV initial accuracy with 0 ppm/°C temperature dependence. Conversely, 100% error allocation to temperature would require a performance of less than 1ppm/°C.