0
\$\begingroup\$

I have recently started using the ADuC702x MCU family. I plan on using the analog to digital converter that comes with the MCU. The datasheet states that there is an internal 2.5V that it uses for its bandgap voltage reference for its ADC. I would like to use 0V to 3.3V for my ADC application.

On the datasheet, page 50 in the Band Gap Reference section, it states that an external reference input on the Vref pin can be used but must be able to overdrive the internal 2.5V.

At first glance, I figured I could simply connect to my 3.3V rail to Vref to "overdrive" it and program REFCON = 0x00. However, the term "overdrive" has bugged me. Furthermore, I have done some research on this and all online schematics show using the internal 2.5V reference. If it was easy to change Vref, I figured I'd see something other than 2.5V being used.

Has anyone ever used the ADuC702x family? Any help and guidance would be appreciated. Thanks!

Datasheet Link: http://www.analog.com/media/en/technical-documentation/data-sheets/aduc7019_20_21_22_24_25_26_27_28_29.pdf

\$\endgroup\$
0
\$\begingroup\$

I see nothing in the datasheet that indicates what "low drive capability" means, but it's probably pretty low, hundreds of uA or a mA or so. The source impedance is given as 70\$\Omega\$ but that does not reflect drive capability.

It's usually a bad idea to use the supply voltage as a reference for the ADC because it's typically more noisy and less stable than a "real" reference. That's probably why you don't see others doing it.

Typical ADC performance graphs up to 3.3V are provided and it's clear that voltages up to AVdd are acceptable on the reference input pin, so I don't see any problem in connecting it directly to AVdd.

I used that chip many years ago, nice to see it's still being made.

P.S. My reading is that you ought to set REFCON to 0x01 to apply an external reference.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.