ADC range

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spuffock

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It seems that I could feed the outputs from the pwm to the reference inputs of the adc, with plenty of filtering, and change the range of the ADC, if I were in no great hurry about it. A full scale range of a volt seems intuitively possible, but putting a millivolt of reference in and expecting it to count microvolts doesn't! Has anyone any experience of just how far you can go and still obtain useful results?
Sorry,I forgot to mention I am using a PIC16F887, but I expect the limitations would be similar for any microcontroller.
 
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hi,
The lowest +Vref recommended with a +5V supply is about +2.5V.

EDIT:
The -Vref can be as high as 2.5V, but from my experience you must have at least a 2.5V separation between +Vref and -Vref
 
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There is a limit to how small the Vref can be for a PIC. Typically, a PIC with 5V Vdd should not have a Vref below 2.8V (this is from memory). I have used 2.5V successfully but it is not recommended by the datasheet.

With respect to PWM, the PIC is capable of producing 10-bit resolution. You can create a linear 0-5V DAC directly from the CCP pin. Just put a small RC filter to help kill the fundamental frequency and it works a charm.
 
Not quite as much as I had hoped for, then. It still looks like a couple of op-amps and digital pots. Unless someone knows better. What I'm looking for at the moment is a software controlled scope input stage, not a lot of bandwidth, 100 KHz is well in excess of requirements.
Before anyone asks, this is an unformed project, much at the stage of:
What are you building?
"A portable"
" A portable what?"
" I've only got the handle"
 
hi,
You can get voltage gain controlled opa's also 64 step digital resistors.
The digital resistors have up/down logic control.
The VCA opa type you could use the PWM method and the digi-resistor from the PIC port pins.
The digi-resistor as part of the gain control resistor network in a standard opa.
 
What are you building?
"A portable"
" A portable what?"
" I've only got the handle"

Good one, I'll have to try and remember that one

Lefty
 
hi,
Have a look at this:

**broken link removed**

and

**broken link removed**
 
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