Another option you may have is that on some PICs the A/D converter has a Vref.
Once you find the expected voltage level (say, 0-2.7v) you can input 2.7v into
Vref+ and 2.7v will read full scale on the A/D. This will also eliminate the
noise an opamp would introduce.
Right now I am assuming you are using a 7805 or similar regulator for your
circuit, which means your supply voltage (and therefore Vref right now) can vary
up to 1%(or more). This is reducing your accuracy by about one bit. If that is
acceptable, you can get 1% resisters and simply make a resister ladder for
Vref. You could then read the Vref with a meter and use that value for your
calculations to keep close to 1%.
I hope this helps.
-Adam
Russell Farnhill wrote:
{Quote hidden}>
> Hi all,
>
> I want to measure current used by a load using an ADC.
>
> The load is a R/C Servo (Hitech 605BB). The ADC already
> exists on a micro controller which takes a voltage of
> 0-5V giving me a reading of 0-255. My idea was to put
> a 1 Ohm resistor in series with the servo power supply
> and try to measure the volt drop across it. I guess
> that the volt drop would be quite small and would need
> an op-amp of some sort to make the voltage change big
> enough so the ADC could measure it. This is where I need
> help as I don't know how to configure an op-amp in such
> a way. I've tried bashing out a circuit in a demo version
> of electronics workbench but without much luck.
> Can anyone help in the way of a circuit diagram or
> something. The op-amp would preferably only need one
> positive supply rail and not a +/- supply if possible.
>
> Any help would be greatly appreciated.
>
> Thanks,
>
> Russ.
cmcorp.com
