Having Problem with External Reference on UNO 32

Created Fri, 02 Dec 2011 16:26:29 +0000 by amir_aj67

amir_aj67

Fri, 02 Dec 2011 16:26:29 +0000

Hello everyone, I am trying to use the External Voltage reference to change it to 2.5 volts instead of default 3.3 volts. The problem is the voltage drops to 1.8 volts when I connect the external 2.5 volts input to it. I am powering the board with USB and the external input is powered with a separate power supply. the code is fine, when I change the code to have the reference as default the voltage connected to that pin stays 2.5 when i change the code to external input it drops back to 1.8 volts. I do appreciate if someone can tell me what is going on in here!

Jacob Christ

Fri, 02 Dec 2011 18:33:52 +0000

What chip are you using for the reference? If the chip is a reference, some have very low current driving capability. What pins on the PIC32 are you trying to drive with the ref?

I'm not sure what exactly is going on inside the PIC32, but I know the AVcc pin has something to do with the core voltage and may draw more than a reference can output.

Jacob

GeneApperson

Tue, 13 Dec 2011 07:07:45 +0000

@amir

What is providing the 2.5V reference source? Do you have any idea what the source impedance of the reference source is? Where are you measuring the 1.8V?

Table 29-34: ADC Module Specifications in the PIC32MX3XX/4XX Family Data Sheet lists Iref (current drain) as typically 250uA with a max of 400uA (micro amp).

You're getting about 0.7V drop. So if the drop is the result of source impedance in the reference, this indicates a source impedance of about 3K ohm.

I need more information about what your circuit looks like to have any notion of what may be going on.

Gene Apperson Digilent