ADuC702x MCU bandgap voltage reference question

Question

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

Answer 1

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.

Answer 2

My suspicion is that the \$70\Omega\$ is in the way whether the reference pin is input or output. The internal reference appears to be derived directly from the pin rather than behind the \$70\Omega\$. If the pin is used as an output or input for the reference, then any current on the pin will affect the reference voltage used by the ADC.

1. When used as an output, buffer with a unity gain amp, so that there is no current flow through the \$70\Omega\$.
2. When used as an external reference source, make certain the source is a voltage source with enough current to supply \$ \left(V_{refext}-2.5\right) /70\$. I speculate that this is the current draw when applying and external reference. You should measure.