ADC voltage-reference errors impact full-scale conversions

SAR (successive-appoximation-register) analog-to-digital references have more influence on conversion accuracy than you may initially think. Figure 1 shows the transfer function of an ideal 3-bit ADC and the same converter with gain error. The transfer function of an ADC is equal to:

where D_CODE is the digital-output code, V_IN is the input voltage to the converter, V_OS is the converter’s offset voltage, V_REF is the reference voltage applied to the converter, N is the number of ADC bits or the ADC resolution, and V_GE is the combined ADC-gain error, reference-output-voltage error, and reference-voltage noise.

It is easy to see how the voltage reference’s specified value affects the ADC’s absolute accuracy. For high-resolution converters, the reference-offset error is usually greater than the ADC-offset error, particularly over temperature. You will also notice from the transfer function that the reference errors have more influence on the converter results with higher input voltages.

The reference’s noise error is a different matter. It affects the SNR (signal-to-noise ratio) and the THD (total harmonic distortion) of a conversion. The reference noise impacts the converter’s SNR and THD at higher ADC input voltages (Figure 2).

If the converter lacks an internal buffer at the reference pin, you will notice incoming or outcoming current spikes. The converter uses these currents during the conversion cycle to charge internal capacitors. This knowledge may motivate you to insert a low-noise amplifier between the external reference and the ADC.

Don’t try to test your ADC with an input voltage of 0V or ground. If you hope to see the effects of your voltage-reference source on your conversions, try to use a dc full-scale input and then a signal input that will help you look at the system’s frequency response (Reference 1).