I've been doing this for years, but got to thinking about describing it here when replying to the "Thoughts on a Unique I2C-DAC Interface" thread. I'm just putting this out there in case someone would find the concept useful.
The minimum voltage specification for the 28x2 vref pin was too high for my needs -- I needed a vref that was very low to handle the extreme low end of a current control range.
I had a requirement to accurately deliver current across a wide logarithmic range from 10ma to 10 microamps with accuracy. I found that I could use the A.3 vref pin far, far below the specified minimum and, since I've now done this across quite a large number of 28x2 chips, I'm confident that it's a characteristic of the chip to support this even if it's outside the specification of the chip.
I have divided the current range into two adc10 ranges, one specified by supply voltage and one specified by the vref voltage that met my needs. The vref voltage I use on pin A.3 is .297v, using a 316R and 4.99K voltage divider off a 5V supply.
Using both ranges with auto-select code it feels like I've achieved something like 12-bit control resolution using a 10-bit chip but I can't do the math to figure it out theoretically so maybe I've got it all wrong. I can, however, confirm that I can control the LEDs in LDRs to accurately drive the LDRs between 25 ohms and 50K ohms with great precision.
Bottom line, I think the 28x2 vref is far more useful than the chip specification suggests, and folks should be willing to try their projects needing a vref that is well below the chip specification.
The minimum voltage specification for the 28x2 vref pin was too high for my needs -- I needed a vref that was very low to handle the extreme low end of a current control range.
I had a requirement to accurately deliver current across a wide logarithmic range from 10ma to 10 microamps with accuracy. I found that I could use the A.3 vref pin far, far below the specified minimum and, since I've now done this across quite a large number of 28x2 chips, I'm confident that it's a characteristic of the chip to support this even if it's outside the specification of the chip.
I have divided the current range into two adc10 ranges, one specified by supply voltage and one specified by the vref voltage that met my needs. The vref voltage I use on pin A.3 is .297v, using a 316R and 4.99K voltage divider off a 5V supply.
Using both ranges with auto-select code it feels like I've achieved something like 12-bit control resolution using a 10-bit chip but I can't do the math to figure it out theoretically so maybe I've got it all wrong. I can, however, confirm that I can control the LEDs in LDRs to accurately drive the LDRs between 25 ohms and 50K ohms with great precision.
Bottom line, I think the 28x2 vref is far more useful than the chip specification suggests, and folks should be willing to try their projects needing a vref that is well below the chip specification.