4 VDDA as Reference Voltage for ADC

For C2000 devices without an internal reference, the cheapest option for VREFHI is to use VDDA as the reference. For C2000 devices with an internal reference, the internal reference is always a better performance option than using VDDA as the reference. Also, some low pin-count C2000 packages have VDDA as the only option to use for a voltage reference. In theory, it is possible to achieve good performance with VDDA as the reference, but in practice there are some common design decisions that can prevent this.

A common design decision that can limit performance is using an inaccurate low-dropout regulator (LDO) to directly supply VDDA, as many of these ICs supply an output voltage with a relatively large margin of error. A 10% margin of error in VDDA directly causes a 10% gain error in conversions. That results in a 400 LSB error for a full scale conversion!

Another key specification for the VDDA supply is transient response. Since VDDA is used to supply power to multiple analog peripherals in the device, there can be frequent changes in the load current sourced from VDDA. If the VDDA supply is not able to quickly resolve to the target voltage, this can cause large ADC conversion errors and overall poor dynamic performance. This problem can sometimes be alleviated by choice of capacitor at the VDDA pin, so make sure to carefully read the output capacitor recommendations for the IC that is supplying VDDA.

When using VDDA as a reference, it is possible to carefully design the input circuit so that ADC conversion errors are minimized. Key considerations for the VDDA supply circuit are DC accuracy, temperature drift, and transient response.