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A common use for voltage references is to provide a stable reference voltage for data converters. Since all input signals are compared to this reference voltage the accuracy of this voltage is critical to the precision of the signal chain. The graph on this slide shows an example of a 12-bit analog to digital converter. A voltage of zero at the input will result in the output code of 0x000 plus a voltage equal to the reference voltage of the ADC, in this case 4.096 V, resulting in an output code of 0xFFF. This is only true if the reference voltage stays stable at exactly 4.096 V; if the reference voltage changes it will affect the transfer function of the data converter. Imagine that there is an input signal of 2.048 V and a reference voltage of 4.096 V. The output code can be calculated by dividing the input voltage by the reference voltage and multiplying it by the number of bits of resolution of the data converter. A correctly working ADC will show an output code of 0x800 (hex) or 2048 (decimal). Now, imagine an unrealistic high noise of 2.048 V on the voltage reference. Although users will never see such a high noise in a real life system it will help them understand how the noise affects the output code. If the input voltage of 2.048 is again divided by the worst case of the reference voltage, in this case 4.096 V, 2.048 V = 2.028 V, the result is an output code of 0xFFF. As can be seen from the transfer function the error from a wrong reference voltage is relative to the input signal. For shorted inputs or 0 V at the inputs there is no error from noise or temperature drift of the voltage reference.
