All analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) require a voltage reference (usually a voltage). This voltage reference is an essential block in ADC and DAC systems. Some converters require an internal reference, some require an external reference. Knowing this, how do we best choose a suitable reference type of ADC or DAC in the application?
In general, there are 3 main reference types to select - internal, external and supply.
Some voltage references are built in the converters. In the below typical circuit, an internal reference type can reduce number of parts in a circuit design, easing design issues. However, it will be influenced by environmental temperatures. These temperature change will cause the voltage reference to drift and affect the converter’s stability.
An external reference uses the applied voltage (or current) as the reference signal for the converter as in the below typical circuit. It allows the design to be flexible.
For example, in high-resolution ADC applications, engineers can use low-noise and positive /negative reference (+/- Vref) (if needed) for the system to achieve noise-free code resolution. It can also add temperature compensation to the system to improve reference stability. Of course, the increase in parts count and design complexity, and their associated costs, is an issue to take into consideration.
The advantage of using a supply as a reference is that any power supply noise is directly coupled to the power supply. This is equivalent to isolating the device from any power supply noise.