about 908AP32 AD

[lijiandong]

Now  a project need 1V-2V  voltage AD samples, i use 908AP32 ,my question is whether may  I connect the Vrefh port to 2V and Vrefl port to 1V for sampling? if Yes , Whether the the  resolution in 1V-2V increases because the span becomes samll, if not, how to solve it?thanks

[lijiandong]

thanks.

appreciate your response.

[rhinoceroshead]

If you're already going to be adding external parts, it would probably be easier to amplify the signal by 5 and add -1 volts with an opamp and a few resistors.  Then you could use the full range and precision of the ADC and not have to write any software to interface the external ADC.  And if you use an external ADC, you still won't get to use its full range anyway.

[rocco]

Hi, lijiandong:

The AP32 is a 5 volt part, and the Vrefl and Vrefh pins need to be at the same voltage as the rest of the chip (don't ask me why; it just does).

The good news is that the ADC has 10 bits of resolution, rather then the 8 bits of many HC08 ADCs. If your reference voltage is precisely at 5 volts, 1 volt will convert to 205 and 2 volts will convert to 410, giving you 205 steps. You will have just under 5 millivolts (actually 4.88 millivolts) per step.

If that isn't enough resolution, you will need to use an external ADC.

Message Edited by rocco on 04-12-200611:00 PM